Coreopsis Verticillata "Moonbeam" will do well throughout the summer however any perennial needs a rich humus soil to thrive.

Here is a link that might be useful: Propagating Perennials

Pretty much any of the real western perennials including the infamous gaura. Agastache would be another one to try, and even the rudbeckia and coneflower old reliables.

There is an entire list of plants that Pennsylvania is generally too wet to grow. For the novelty factor, that's where I'd start.

There are a great many plants for sun and very dry soil that will bloom a long time, but not for only 4 hours sun. I think any of those mentioned will do only moderately well with only 4 hours sun daily. That is a shade situation, although if the sun is from noon to 4, it becomes a very difficult situation. I'd go for foliage and consider plants like Lamiastrum 'Herman's Pride', possibly Hellebores, possibly a small shrub like Ceanothus americanus--New Jersey Tea, and possibly columbine for spring bloom. Iberis--perennial candytuft--will do well in dry soil and only partial sun, and will give great early spring bloom. It's evergreen, but certainly won't bloom after its spring bloom.

Can't beat Walker's Low nepeta for continuous bloom. Shear back once during the summer and your blooms will take you well into the fall. Even when not blooming, I like the gray foilage

My first thought was delosperma (ice plant) but as laceyveil says, with only four hours of sun, I'm not sure it will be enough sun.

Maybe something like a groundcover, perhaps sweet woodruff, and then one of the shrubs laceyveil suggested...?

Good luck!
Dee

Nepetas are wonderful perennials and they love dry soil, but they won't be happy with only 4 hours of sun.

Nepeta's like lots of sun! Mine had more than 4 hours but I like full plantings with little space between plants (so plants shade each other near the soil line) and the plant was very floppy. I got rid of it! Nepetas need more sun than iris (in my personal experience). This of course may depend upon cultivar

Please don't tell my nepeta it isn't supposed to grow in the shade, because it obviously hasn't read that book. The places it has volunteered are definitely much shadier than anywhere I would think of putting it. The flopping is a cultivar characteristic, and not a response to shade. I know some in absolutely all day sun, no shade at all, that have to be cut back in July because they've opened up so much.

I agree regarding nepeta. Ice plant (delosperma cooperii) is also great and a groundcover. Lavender works for me as does salvia and agastache.

Cameron

Here is a link that might be useful: my gardening blog has pictures of these