Be aware that a mail-forwarding service may not be the best bet. They are easy to set up, and a forwarding order at the Post Office will have all of your mail sent there. However, when you want to go back to your old address you CANNOT just fill out a forwarding order at the PO. You have to contact everyone who sends you mail at the service address and have them change your address back. For four months it probably isn't worth it.
You mentioned having your friends in CA do the work for you, and that may be your best bet. You can then fill out a temporary mail forwarding order to cover the four months you will be traveling, and then your mail will automatically return to being delivered at your home.
Since your friends aren't really in the mail forwarding business, covering their out-of-pocket expenses would certainly be legal and right. Rather than pay them $1/mailing or whatever, you might want to just take some friends out to dinner at a nice eatery, or take them to a show, or something like that that they would enjoy.
David Lininger, kb0zke
Heartland mpg 181 (sold)
1993 Foretravel U300 (for sale)