I got my battery kit replaced this past weekend. It's something that Apple does cover if you still have their 1-year warranty. I would say 4.5 hours of usage is unacceptable and an Apple Genius should be able to run diagnostics on your phone and determine how bad of shape the battery is in. If it turns out anything like mine, they'll offer to either replace the battery or replace the phone entirely.
I really wasn't picky, but honestly I was hoping for a complete phone replacement, mainly due to me just wanting any excuse for a free new iPhone. That was almost the case, but he offered to replace the battery for free, which isn't a bad scenario either. It took about half an hour and my phone was ready.
I'm now up to 9 hours of usage on my phone, so I couldn't be happier (well, maybe I'd be happier with a new phone, but hey, 9 hours, c'mon, that's pretty damn good).