what is the life span of iphone 4 battery

The design lifespan, for Lithium batteries like the one in your phone, is 3 ~ 5 years.

That question has no meaningn out of the contect of use. It would depend on how many full discharge cycles and Apple hasn't published that spec.

The battery is under warranty for 1 year. So, in a sense, Apple is saying it should last a year. Will it last longer than that? Quite possibly but without knowing how much you use your phone, it's impossible to say.