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

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.

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