for 250g tank I'd put 15-20 in there, what's the dimension of your tank anyway? as long as the width is wider than 18" you'll be good. make sure your filter can keep up as well
see if you can get some caribes, those are bad ass. Shark Aquarium has them in now. get some while stock last, I think it's like once or twice a year kind of thing.
email G at info@sharkaquarium.com
what's this based on? Larger shoals ALWAYS look better but what facts support that 4 is better then 3 other then the theory that with a large enough tank you can overcrowd it a lil bit to keep aggression down.
3 in a 55 would give them room to each have established territories.