Welcome to R2R! Could get a basslet, dartfish/firefish, goby, pygmy wrasse, dwarf hawkfish, cardinal, small blenny, etc...
Lots of choices :) I think in a 30 gallon tank 4 fish is pretty close to maxed out. If some of them are very small, like neon or clown gobies or barnacle blennies... might be able to get away with 1-2 more, but you’ll be running short on territories and clowns tend to try to claim the whole tank once they start breeding in smaller tanks.
I had a single clown, a yellow watchman goby, royal gramma, and a yellow lined cardinal when I had my 28g bowfront.