had the same problem you must be jailbroken to fix it you have to use sbsettings i think or activation or cydia whichever is your choice and delete things that might've caused the error. i don't remeber exactly but it's something interfering that you downloaded in cydia. delete recent tweaks until it works.
I'm not sure if this is what your looking for but here you go . http://support.apple.com/kb/ht3939#iphon... http://www.ebay.com/itm/iPhone-4-Replacement-Back-Glass-Black-/190832576213?pt=US_Cell_Phone_Replaceme... that's for verizon but you need to be phone specific because at&t verizon have different backs.