Re: What is the average life expectancy of the iphone 4?
almost anything can be replaced when you know what youre doing. you can open the phone up and take some stuff out but i do not recommend it. if you don't know what youre doing you can royally mess up your phone. but to answer your question, yes it is possible but i would bring it to a professional to replace it if you find a new one online.
Yes, the battery can be replaced. You can either take it a nearby Apple Store, an authorized Apple repair center or do it yourself using an online tutorial like the one from the iMore Blog via the following link.
Welcome to iMore! Apple charges $79 plus $6.95 shipping for battery replacement. Batteries - Service and Recycling - Apple
I'm not sure it's worth the expense for an iPhone 4.
100% agree with this. If you feel you really have to though I would go through an authorized repair shop to ensure you get a real Apple battery and not some cheap knockoff. Just my two cents.