How to reset an iPhone 4s to factory settings without knowing the apple ID and password?

I bought an iPhone 4s off a girl a few days ago, turns out it's activated under an apple ID and she doesn't remember the email or password… I can't even get onto the home screen to press settings or anything, after I Sign into my Wi-Fi the next page says " Activate phone. This phone is linked to an apple I d. Enter the ID and password that were used to set up this phone" I hope there's a way I can bypass that? I would hate to waste money on a phone I can't use.

You can't. Unless she can get the password (assuming she's even the owner) nothing you can do.

Unfortunately it is very likely you got ripped off by someone selling an iCloud Activation Locked iPhone.