Not sure about the i-phone 4 but the 3GS has a pathetic 1200mAh battery.
Standard Desire battery is 1400 but you can get a spare "better" one for a small outlay. With the i-phone that it – battery is integrated.
I've still vaped my battery in under a day on a recent trip to London (on the train, used it a lot) – best tip is when out and about, switch off all auto synching – pretty much doubles the battery life.
Got mine direct from Orange but it's on a business plan. Paid £100 up front, pay about £35 a month with a big wad of free MBs chucked in. Not a great deal but okay for me.