typo? a 4' + fish in an 18" deep tank?you would be able to keep them perminantly in a 75.
some porthole cats would be ok in that size tank, they get 8"-9", lace cats get 6"-7".
I'd go Here and check out the shape you like most, then you can explore their sizes to find something that will fit your tank. I've got a 240G and even though I like them, I'm avoiding bullheads because in the long run, my tank isn't big enough to give them a decent life. maybe a spotted if I run across them, they would be OK in my tank.
I wouldn't go above 12" in a 75G (48"x18"), and even then I'd be hoping it didn't grow full size. 8"-9" long is a good size for that tank.