Cresties are arboreal so unless the 4 ft is the verticle measurement, its not ideal. It would probably do but they'd be happier with some height. a 2ft cube is probably best.
couple of points:
1. All cresties in the pet trade came from the same small amount of animals originally shipped over from New Caledonia so you're extrememly unlikely to find 2 completely unrelated, however, most descent breeders will have some idea of where the parents came from so ask around.
2. As they're going to be your 1st lizards, i'd have to recommend you get just females, at least just till your comfortable with their day to day care. Cresties are so prolific that they will breed and its very easy to get overrun- and with a trio you will get twice as many eggs! Hatchlings will need to be housed separatly from each other to ensure you can monitor them individually and then you'd need to find someone to buy them.also, be careful when choosing a 'probable' sexed gecko, some males 'drop' very late (its best to use a 10X jewellers loope to look for herpimenal pores instead of just the bulge) and i know SO many people buy probably females and have them turn male. Females should not be allowed to breed until at least 30-35gm (about 14months old) so you wont be able to introduce the male until their up to weight.
3.. There shouldnt be too much problem with fighting, although i have heard of it so you would have to watch out carefully when you introduced the male, and of course give him a quarantine period of about 90days before letting him anywhere near your females just incase he has anything.