You're best bet in my opionion is to give them or sell them to a friend. I'd be really hard to expect someone to take care of them for that long, just to hand them back over. You can try to write out a contract with a friend if you plan on getting them back, whenever that may be. All my friends that have joined the service have stayed in for longer then 4 years, so it may be hard to keep them getting sent different places all the time.