i was given an old iphone 4 from a friend how do i reset it to factory

Go to settings, general, and scroll down until you see reset.

Even if you are successful reseting the phone, you will find it will not run virtually ANY modern apps. iPhone 4 is NOT LTE. This means it is a 3g phone and in the US most if not all carriers have already shut down 3g service.

You have a pretty worthless, useless iPhone at this point, which is truly ready for the recycle bin.