I dropped your zone, size requirements, full sun, into the search at the site below. I assumed "moist but well draining" since you have a hill.

A couple smaller maples popped up. I would think you can do better than the Box Elder they list though lol

Here is a link that might be useful: Hort.uconn search page

ha... box elder... I spent all last summer chainsawing those suckers out of my back yard ;)

Yeah Box Elder is up their with Bradford Pear and Russian Olive and a Cottonwood not in a natural situation (kept in the wild).

I have found that this far north I can often push the envelope with sun requirements. I grow several dogwoods in full sun, although they tend to be trees that prefer to grow in part shade. I have 4 Cornus alternifolia (pagoda dogwood, a native) and a Cornus kousa (kousa dogwood) all in full sun along with some in part shade. The full sun plants tend to be denser and have more flowers. The kousa won't be hardy enough for you, but the pagoda dogwood will be. It has a lovely winter silhouette of dark gray, layered branches, creamy Queen Anne's lace type of flowers (not what one normally thinks of as dogwood flowers) in spring, and clusters of small berries that ripen through red to dark purple with bright red stems that the birds love. Fall color is yellow and pinky-red, rather like an almost ripe peach. It can be multi-stemmed or single stemmed. Truly an all-season tree. There are some with variegated leaves (both green/white and green/gold) but I wouldn't recommend those for your location since they would probably burn.
{{gwi:465835}}From garden photos 07
{{gwi:465836}}From garden photos 07

Other possibilities to research that I've seen growing near me or have grown myself: smaller disease resistant crab apples, the hardier strain of redbud from Bailey's nursery, Hawthornes such as Crataegus viridis 'Winter King', Yellowwood (Cladrastis kentukea). Remember that since you are so far north, many trees such as the yellowwood may not reach their maximum possible size for many, many years, if ever.

Regardless of what you plant, I'd give it a good layer of mulch (though not right against the trunk) out to the tree's drip line to keep the soil cooler and more moist to mitigate the baking sun effect. Proper watering will also be needed for a tree to thrive on a hot slope.

I've linked the Missouri Botanical Garden's search page as another good resource in addition to the U Conn hort search linked above.

Here is a link that might be useful: Missouri Botanical Garden search

Being a st louis boy you would think MOBOT would be the first thing i would recommend lol. I was even planning my fall visit!

Thanks for feedback guys! nhbabs, local nurseries do carry dogwood (not sure what kind), hawthorn and redbud. I personally think hawthorn is ugly. We had considered redbud and the spring flowers are great, but supposedly not long lived. I hadn't considered dogwood and maybe I will check that out in more detail.

A south-facing slope that "gets baked" probably means hot and dry summer conditions, so birch and pagoda dogwood would probably not be the best options unless you plan to water regularly (as nhbabs pointed out, "proper watering will also be needed for the tree to thrive on a hot slope").

Considering the other factors (zone 4, clay soil, possibly ~high pH?, max 25' but large enough to provide shade), you may have only a handful of alternatives. One of the smaller drought resistant maples or one of the larger crabapples would probably work. Maybe "Pacific Sunset" maple or Tatarian maple. "Spring Snow" (seedless) and "Selkirk" are a couple relatively large crabapples.