I've been experimenting with this height for edging, and have not found anything that fits your requirements perfectly. I considered gold leaved plants for their foliage, like golden origanum or golden marjoram. Or lime/golden lamium that has golden leaves, but not always flowers. Slightly higher Geranium Rozanne Crannesbill, is a winner though. Also catmint, there is a dwarf version, also a bit higher than 4 inches. I tried scabiosa and campanula and pink coreopsis, but non of them flower all summer.
What about annuals? There will be quite a lot.
I'd love to see what the other gardeners say here.

a number of creeping plants will meet your height requirements however flowering all summer long wlll be a problem. The only solution is using annuals like petunias. Or if you are into succession gardening, you can plant perennials creepers that blooms at certain stages of the season. For example now blooming are campanulas and speedwells. in a couple of weeks my thrifts would start blooming. later some dwarf form of lavenders will bloom all summer. you may then consider some low growing achilleas (careful they spread).

I've grown oregano btw. Great spreader but their flowers bloom tall and flop over.