Easiest way for two people to switch an iPhone 4 and 4s
Connect both phones to their current respective computers
Open iTunes and perform a backup on both phones
Swap the phones to the opposite computers.
Once swapped, click on the Restore button in iTunes, then click
Don't Back Up.
When iTunes finishes the restoring process, set up your 'new' phone
from a backup, making sure to click on your old phone's latest
backup (the one you just completed.)


This should restore the new phone to your old phone's last condition.