How to use a old iPhone 4 without going to the phone store?

So I have an android (I have for abut a year or two now) and I have been wanting to try something new. My brother has upgraded to a iPhone 6 from a iPhone 4, and I wanted to try it out for a few days. Is it possible for it to still work like if I put my email in or will it screw up his new phone if I turn it back on again?

It would work fine, as long as he cleared all his data from it. You could put in your data & it wouldn't affect anything on the new one.