How can I erase my iPhone with no internet connection?

Hi, I have recently updated to iPhone 5S and now want to pass on my iPHone 4S to our son. I am still awaiting the activation of the new sim card for the 4S and therefore have no way to connect to the internet. When I try and delete the phone through the phone settings, it asks for my iCloud acount password, but as I am not connected to the internet, nothing happens.

Would it work if I used my sim (from the 5S) in the 4S phone and then delete? Or should I just wait for the new sim to be activated?

Thanks

C