Bottom line, if your battery life under normal use (wifi off, listening to music) is around 4 hrs, demand a replacment if its under a year old,
If your only getting 4hrs battery life but your playing infinity blade and watching HD movies, then i think you may not have a case.
I found that apple has a terrible issue with battery life in all its products, they seem to be fantastic for a month or so, then suddenly fall off a cliff.
Example, Iphone 2G : bought on day of UK release, still being used by my mother, holds a charge for two days, and is still usuable as a phone (around 20% battery left after 30 hrs) light use checking emails, making calls etc.
My Iphone 3GS, when new, 12hrs light use, 10hrs when playing games/video, after 7 months, 6hrs with light use, dead in 4 if used to listen to music, and about 2hrs life if used to play games.
even my Macbook pro battery died after only 36 cycles, 5-6Hrs battery life for the first 5 months, then month 6 it fell of a cliff, rapidly losing 30 mins charge per cycle until the battery reported it was dead.
Which is why my new phone is a Samsung Galaxy SII, 24hrs battery life with moderate use, and i can replace the battery myself in about 3 seconds.
bottom line, take it back, take the recipt, and quote the sales of goods act 1979 (as amended), an item should be expected to be fit for the purpose for which it is purchased, yours is obviosly not.