There are 4 week assignments for all specialties; being an OR Nurse is a big plus. They are not nearly as plentiful as 13 week assignments, though. You do need to "ask" all the Agencies you come into contact with about this option and tell them off your interest. Let them know, you also intend to renew if that is your goal after some time off.
I have also heard that you "can" sometimes negotiate time off (not paid and occasionally paid) in between the period of your contract. Be creative, keep good notes of your research, and remember how valuable you are.
What state, nurseontheloose, are you in? Is it possible to work a drive away from home?
There are several Agencies who utilize 4 week assignments, that I am familiar with: Travel Nurse Solutions, Fastaff, NurseChoice, and Critical Nurse Staffing. There are others but it depends on what state you want to go to for information to an area.
I have also called facilities, spoke with the Scheduling Office, to see who they use for Staffing with an Agency. It helps tremendously if you know where you want to go. I have spoken with the Nurse Manager and had them review my information before an Agency was able to recruit me. This is a lot of extra work but I like getting what I want.
Good luck to you and let us know how you make out.