[gregzz4]

This is only an assumption based on most fluorescent ballasts I've dealt with, and not just aquarium ones ...

Looking at the #'s I'd guess the one labelled 2S54 is your 2-bulb, and the 4S54 is the 4-bulb ballast

Best way is to trace the wires inside the fixture - backwards from the bulbs that are out - to the ballast
Another way is to disconnect the power to the fixture, unhook a ballast and plug it back in

Not sure you'll find a T5HO ballast at a hardware store
Last time I looked they were T5 only

Good luck !