You need to get a sim sim, activated, wirh your wife's account info on it. Your carrier should give you one for free. Either that, or you cut down your wife's existing sim. If you do a Google search you'll find instructions/tools on how to do so.
After you get the sim situation straightened out, backup your wife's 3G in iTunes. Then on your 4, turn off iMessage, FaceTime, Find My Phone & delete the iCloud account. Then: Settings>General>Reset>Erase All Content & Settings. When finished, restore from the backup you made of the 3G. Sync her content to the phone.
You should be good to go.