My iPhone 4 (and my previous 3GS) have/had a neutral, bordering on cool colour temperature (tending more to blue than yellow). Apple often often source screens from different manufacturers in their products so you will find variance. If it's not to your liking, I'm sure genius bar will do a straight swap on a brand new unit. You could also compare a couple in store to see if yours is unusually warm.
When I got my 3GS replaced by apple it had yellow Tint .I rang apple up and complained to apple on the phone and they replaced it with a nice crisp White one (well blue one) I hated the yellow tint it looked like it had been tobacco stained. Oh and a £40 spend on the apple store for the problem