It all depend on the condition of your existing garden soil. Tomato is not any different from other garden veggies. But having said that, adding compost should always be good as source of some nutrients and as a soil conditioner.
About the pH : Again it depends on what the existing pH is. Normal compost is not going to make a drastic change. Besides, tomato can thrice in wide range of pH (5.8 to 7.3 ?), The optimum/ideal being 6.8.for 99% of home garden veggies.

mix some other source of potting mix into the ground when planting.

There is never any need to add potting mix into the garden ground. Not that adding it hurts anything as many will mix old used potting mixes from containers into their garden soil rather than throwing it away. But is isn't required for any reason.

Adding other soil amendments and balancing the pH all depends on the native soil and for that you need to have a soil test done.

You can always add good quality compost of course with no problems but when you get into other additives you can often make the soil worse if you don't know what you are doing.

As seysonn said, tomatoes don't require anything different than most any other garden vegetables does so why not do some reading over on the Soil & Compost forum here for the best info on how to improve your soil if needed.

You can also contact your local county ag extension office for info on what your native soil is like as soil type, consistency, and quality varies widely in different parts of the country.

Dave

Thanks guys. I appreciate the help. Sounds like I don't need to make too many adjustments. Any ideas to what I can do to keep a higher yield? I don't want to lose any potential of my plants

I don't know of any trick, secrete or special way to increase "potential" of tomato plant, other than just normal care. But since you are in USDA zone 4, I can tell that you have a relatively shorter growing season.. Say, your tomato season begins by late May/early June and ends late September. That is about 4 months(Effectively). The median of tomato DTM (from planting seedling to first ripe fruit) is about 80 days. So you have just 40 days (~6 weeks) to harvest ripe fruits. How can you increase/extend that 6 weeks to 12 weeks ?: THAT IS THE REAL POTENTIAL, I think. I am almost in similar situation and am trying to take that challenge, by more than one way. Here is my strategy:

--- select varieties with shorter DTM. (55 -75 day)
--- select more DETERMINANT types, that grow less foliage.
--- select cool weather tolerant varieties.
--- Extend your season by trying to plant early, using W.O.W., hoop,, cold frame, clear plastic mulch

Other than those, I would just take normal care (watering, fertilizing, some pruning ...)

This post was edited by seysonn on Sun, Feb 23, 14 at 17:25

Plant so you don't have to prune. Leaves make energy so the plant can make tomatoes.
So try to space the plants and have wide enough paths to keep pruning at a minimum.
That also means planning for what you are going to do for supports and so on before the day they are actually needed.
I had a ton of cherry tomatoes a few years ago from a plant that ended up sprawling without support, but it was in a raised area where people wouldn't walk on it, and it took up a lot of room (which I don't have to offer to a tomato plant at my current house).

Seysonn- I'm not much of a tomato guy but I'm growing tomatoes so that I can make some great salsa. Are there any tomatoes that you can think of that you can think of that will be good in salsa but also for my conditions of growing?

For salsa you can use most any variety but your short season is your biggest handicap for growing. If you stick with short DTM varieties (aka Early varieties) then consider Grushovka, Siletz, Matina, and maybe Silvery Fir Tree or Clear Pink Early if you are growing your own from seed.

If you are buying transplants then it will all depend on what varieties are available for you to buy locally.

Dave