I have about 25 small pots that hold 2-4 cups of soil each.
Are there some kind of colorful flowers that would be content to spend their life in such small pots? Or how about a radish or some kind of herb?
I'd really like to put these things to use.

Lettuce does well in those. Plant two or three different colorful types each. The Violet family should do well, and succulents.

Some of the smaller marigolds, like Sophia should do well. In four cups of soil I would think you could probably grow a geranium, petunia or some of the smaller mums.

Thank you both for your help. I was worried about root-bound. I already have lettuce planted, but I'll give the others a shot.

Radishes do well in pots. But don't plant them too close together. Probably no more than two per pot in pots that size. The seed packets with a mix of different types of radishes are fun. I think most herbs would need bigger pots.