Ellen,
In my experience, plants have to be "stepped up" into larger pots. For instance if you had a 4" potted plant and planted it into an 8 or 10" pot it would not grow, it would die. But if it was planted into a 6" pot it would thrive. The reason for this is that (for some reason, I don't know) plants will in essence drowned if they are planted in a container that is too large. Is this clear? It's hard to explain.
It's like they have too much room, too much water and they are overwhelmed and they don't grow. It may be that they grow more roots in a smaller container because the roots hit the sides of the pot sooner? Maybe it acts like a root pruning action and spurs it to grow more roots. Thus it thrives.
In anycase those seedlings should have been taken from the seed flat to a 2" pot the the 4" after 4 weeks. That's how I do it. But I will say this- if you have been growing seeds for a long time you probably have seen more than I have.
Just remember they need to gradually step up in size. When I worked at a nursery where we sold 1000's of house plants every year we informed our clients that they should never repot a plat to a new container that was more than 2" bigger in diameter than the previous container. The theory holds for smaller plants too.
As for your plants right now...not sure if this will work but let them grow on the dry side a bit and if you can get a hold of Superthrive, I'd start feeding that too them.
Good Luck!

gardendollee, thank you so much!

I have always known, with houseplants, container plants, etc, not to plant more than one size larger than the original pot size. But so many different plants grown from seed are advised to "transplant to 4 inch pots" or even to sow directly in 4 inch pots (which I never do). I would have never thought that a 4 inch would be too large for these petunias. It never once occured to me there would be a problem. The "stepping up a size" system is what I always did, but I always thought that it was unnessary for some things. But I guess I felt safer doing it that way. I can guarantee you I will not make this mistake again!

Okay, I screwed up. Did I screw up permanantly? What can I expect now? If they are not going to thrive, let me know so I can put an end to my misery! :(

In my personal opinion they can be repotted to bigger pots even more than to just 2", but go slow, dont try to water or feed a small plant that is in a big pot. Sometimes they even get stunted for a year, don´t rush things. That happen to me with a fruit tree and took it about 4 months to start to grow again and when that happened it growed like mad, about 6 feet in just in just 4 months. I think is just matter of time, be patiente and remember is a small plant in a big pot, go easy on water, fertilizer, etc.

I'm not sure I agree with the theory that plants will die if placed in too large a container. By that logic, container plants that are planted in the ground would have no chance. It is a good practice to pot up no more than 2", but I do that to keep the roots in a smaller volume and give a better ratio of top growth to pot size. It looks better when you are trying to sell them.

How lasting the effects of slowed growth are depends on the plant involved and why it is not growing. With annuals, they can start to bloom at a very small size. The strategy of most annuals is to get as big as possible then start blooming before resources run out. If a plant is stressed by lack of water or growing in a nutrient poor medium, then it might start blooming very early. At this point, the energy of the plant is directed into blooming and producing seeds, not new growth. So for your petunias, I would start pinching the flowers to encourage more leaf and stem growth. Also, there is the unanswered question of why the plants aren't growing well. Transplanting petunias shouldn't make them stop growing for more than a week.

I have questions about the growing conditions that will help me understand what the problem is. Do the plants appear healthy otherwise (color OK?)? Take a plant out of the pot and look at the roots. Do they look healthy or discolored and diseased? Do you have fungus gnats? What fertilizer are you using? What potting mix are you using? Have the plants received regular watering? There has to be something that is not right, but I can't say with the information I have.

botanybob, thanks a billion for your interest, as I still have some unanswered questions.

First I must clarify something. I realize that I have confused some things here, as I have this problem going on two different posts in two different forums. First: it isnt the petunias that are blooming, that confusion happened because I told the story about the begonias from last year. I had used that story as an example of the question I had about the possiblitly of plants getting permanantly stunted. Let me stick first with the petunia issue:

When posting this, I had 8 week old petunia seedlings (wave and avalanche varieties) that I had transplanted(4 weeks earlier) to 4 inch pots, rather than cells (72 cell flats) that I would have used for regular grandiflora/multiflora petunias. I made the choice to skip transplanting to cells because I knew that the waves and avs would get fuller than regular petunias, and would be transplanted to 4 inch pots eventually. I usually use promix potting soil exclusively, however, this year I opted to use a promix/miracle grow mixture, in effort to save a few dollars. This shouldnt have been a problem, nor lead to this problem, I have used miracle grow for seedlings before when I couldnt get to the nursery to get "the good stuff". I grew my petunias in a room that gets no lower than 60 degrees at night, about 70 max during the day. Perfect for petunias, right? But maybe notso when they are such young seedlings???? I use a 10-10-10 fertilizer diluted TWICE at every watering.

At the time I posted this, my seedlings were almost 9 weeks old, and barely had 3 or 4 sets of leaves. Barely bigger than they were when they were transplanted 4 weeks earlier!

I belive 100% that my mistake was in transplanting to 4 inch pots. When I did a root check, the roots were growing streight down. They were healthy roots, healthy leaves, still very green and "fuzzy" like healthy petunia plant looks like. They simply were not growing, not developing new leaves. About 5 days ago, I made the choice to get those seedlings out of those 4 inch pots and put them into cells. I did this very carefully. I consulted with a local grower who said that this was the right thing to do, and she said that I can expect those plants to fill out in about 3 weeks, though as if nothing happened. She felt that I was having a hard time keeping even moisture in these 4 inch pots, even though it seemed that I was watering correctly.

Now that I have done this, I am still curious about whether or not these plants will be permanantly stunted. This is the reason for this post, as I cannot find information anywhere that answers this question for me. If a seedling does not get the growth that it should have at a certain time, will an overall growth be affected? In this case, since the petunias did not develop the next several sets of leaves when the seedlings was 6,7,8 weeks old, will there be a permanant stunting of the plant? Or will it simply be delayed? Can I expect these petunias to still grow to the 3 to 5 foot wide spreading plant that it was intended to be, or will it perform more like a regula petunia? These are the questions I need answers for. I guess I am getting into the plant science aspect for it, and that is why I am here.

In the case of the begonias, the same thing has happened this year. I have lots of plants that have fully filled out their cells and have started blooming, seemingly at the right time. These plant look great. And, I have some other, smaller seedlings that were transplanted at the same time, but simply were smaller seedlings that I didnt want to just toss out. They have not filled out their cells, barely have 3 or 4 sets of leaves and they have started blooming as well. I do not know of any environmental stress issues with these plants that would cause them to begin blooming early, they simply seem to be blooming at the time that is "right" for them. So, this makes me even more confused. Will a smaller, undeveloped, plant begin blooming at a certain time, even if it has not grown in the way it is expected to? In this case, the seedlings were extra small.

So, to better clarify my questions:

#1 Can I expect the petunias to be permanantly stunted because they did not develop their next sets of leaves during the 5,6,7,8th weeks since being sown?

#2 Will a smaller, underdeveloped plant begin blooming when it is "time" even though it only has 3 or 4 sets of leaves, and seemingly no stress factors? And, if those plants have started blooming at this time, has the plant reached a different "stage" of its lifecycle that will affect its overall growth performance? Can I still expect those begonias to grow to the way they were expected to?

botanybob, I hope I am making some sense here. I appreciate your help with this.

It is Temperature- temperature-temperature. The larger the pot the more water it holds. If I spray you with a hose and sit you in soil- what will you feel? Cold. Crank the soil temp up and they will grow. Also I would bet you had cloudy days and no direct sunlight to warm the soil up. Size of pot has no relavance to root growth. Soil temp does..
Good luck.

I am quite sure that the petunias will grow normally once whatever conditions that led to this problem have improved. There is no "memory" or permanent effects in herbaceous plants (outside of getting badly rootbound) that I am aware of. I like PaTeeny's comment about soil temperature. She(?) is right about soil temp being more important than air temp. Worth checking out.

I can't really comment about the begonias because I know little about them.