Had to get round to opening mine because of a cracked screen.
(btw, the closest replacement I could find was for a 4, not 4s – i hope it’s the same size when it arrives)
What you need to do is use a heat gun or a hair drier to heat up the two glass elements at the back and pry them apart.
There are two small screws right at the center of top and bottom holding the back panel in place, and it’s glued too, so you might want to heat up the borders while prying it open.
I realize the thread is a few months old but I’m leaving it here in case someone else is looking for it.
Good luck
-d
