My fish died, will the rest be ok?(cories)?

I bought 5'juli cories on 6-3-11 and now, one died all of a sudden. No other fish, just that one. Luckily it is still in the 30 day guarantee so I plan on returning i tmrw and my question is..... I really should be saving up for my new 75 gal tank that I want so bad! I don't have a job( I'm 15 years old) so do I HAVE To buy another juli Cory or will the 4 bE fine with just the 4? It's a 40 gal community tank.