4inches of something called "topsoil" on several feet of sand is largely a waste of your money since that is not nearly enough for a good, healthy turf to grow in. Even if that "topsoil" did have about 6 to 8 percent organic matter in it that would only be barely enough for the grass to root in, 6 inches would be better provided the soil below also had adequate levels of organic matter.
Adequate amounts of organic matter in the soil will aid in retaining moisture in the plants root zone as well as providing nutrients those plant need to grow.
The soil plants grow in is the key. Get that into a good, healthy condition and the turf will grow in and limit the "weed" growth.

Well... that's what's available under the grass, so I need to make the best of it. Any advice how I can make something of this "waste of money"?

And our lawn is pretty nice already. We get many compliments on it, so our "topsoil" must be adequate feed for our lawn. I just want to make sure I'm fertilizing correctly with organic ingredients and boosting the rich green of our lawn.

Anyone with a little more of a detailed useful plan?

In Michigan, theoretically, you need a soil test to purchase any "fertilizer" containing Phosphorus because of the problems the lakes and ponds are seeing from toxic algae. So, start now with a good soil test through your counties MSU Cooperative Extension Service. That can be a good guide about any "fertilizer" that might be needed.
Unless the sandy soil you have has adequate levels of organic matter it will simply allow the moisture and nutrients your turf grass needs to grow to flow through and into the ground water. So, it is a good idea to know how much organic matter is in that soil. 6 to 8 percent organic matter in soil provides a good environment for the Soil Food Web, the wee criters that live in the soil and feed the plants we grow, to live and work in and helps keep adequate moisture levels in the soil as well as sufficent air.
Along with the soil test from MSU that will tell you your soils pH and levsls of major nutrients these simple soil tests may be of some help.
1) Structure. From that soil sample put enough of the rest to make a 4 inch level in a clear 1 quart jar, with a tight fitting lid. Fill that jar with water and replace the lid, tightly. Shake the jar vigorously and then let it stand for 24 hours. Your soil will settle out according to soil particle size and weight. A good loam will have about 1-3/4 inch (about 45%) of sand on the bottom. about 1 inch (about 25%) of silt next, about 1 inch (25%) of clay above that, and about 1/4 inch (about 5%) of organic matter on the top.

2) Drainage. Dig a hole 1 foot square and 1 foot deep and fill that with water. After that water drains away refill the hole with more water and time how long it takes that to drain away. Anything less than 2 hours and your soil drains� too quickly and needs more organic matter to slow that drainage down. Anything over 6 hours and the soil drains too slowly and needs lots of organic matter to speed it up.

3) Tilth. Take a handful of your slightly damp soil and squeeze it tightly. When the pressure is released the soil should hold together in that clump, but when poked with a finger that clump should fall apart.

4) Smell. What does your soil smell like? A pleasant, rich earthy odor? Putrid, offensive, repugnant odor? The more organic matter in your soil the more active the soil bacteria will be and the nicer your soil will smell.

5) Life. How many earthworms per shovel full were there? 5 or more indicates a pretty healthy soil. Fewer than 5, according to the Natural Resources Conservation Service, indicates a soil that is not healthy.

Detailed plan beyond what morph gave you? How about this for details...

April 1 - preemergent herbicide and Milorganite at 20-30 pounds per 1,000 square feet

May 1 - Soy bean meal at 20-30 pounds per 1,000 square feet

June 1 - Milorganite

July 1 - soy bean meal

August 1 - Milorganite

September 1 - Soy bean meal

October 1 - Milorganite

So your wife thinks short grass looks better than this...

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Thank you! I'll show here the picture and I think she'll agree with you.

What would you recommend for a preemergent herbicide?

And Kimmsr, I've done the jar test and will send my soil in to the MSU Extension for analysis.