I'm not all that familiar with tank sizes measured in foot so a 4 foot tank really doesn't tell me much what size it is in gallons or litres, but as the clowns grow big they will require at least a 100 gallon tank, preferably something around 150+ gal. Even while they're young, I wouldn't keep them in anything less than a 55 gal as it will stunt their growth.
I've heard they can get along with a red tail or redfinned shark in a big tank, but I have no personal experience with it since I don't think my tank is big enough for the clowns and a shark (it's a 5 foot long 80 gallon). I'd imagine more clowns would make them more active, being out and playing more. My clowns are pretty funny to watch when they play, zooming across the tank and swimming in circles and upside down at evenings, and I think they would do it more if I had a few more clowns, but I'll wait until I upgrade tanks before I get more.