The odds of finding a color camera that will match your current connector are very small.
If you decide to buy from RV Cams
RV Cams, Inc -- Wholesale Distributor of Premium Backup Camera Systems
I've heard that they have some cable adapters. You could call with your camera model number and see what they have.
I copied the following info from an amazon publication about rear view cameras
• Wiring - Wired back-up cameras have a few common installation considerations:
o Power Connections – Most back-up cameras require a wired power and chassis ground connection. This is usually provided by connecting the camera wires to the vehicle’s reverse light wiring and a metal ground point on the body of the vehicle in the trunk/hatch area.
o Video Signal – The video signal wiring is run to the front of the vehicle where the driver’s video monitor is mounted. For many cameras, the video wire is a composite video (yellow RCA) connection. Some cameras use a multi-pin cable that contains both power and video signal, which eliminates the need for additional wiring connections in the trunk.
Adding to the info above, I suspect your 4th pin is to provide power to the camera from the monitor so that you can power on the camera when the vehicle is not in reverse.
If you don't want to run a new cable, you could order the replacement camera, see what connectors it uses for the power/ground and the video signal, and then go to an electronics shop(like Radio Shack) and buy matching connectors to solder onto your 4 pin cable after you cut off the existing connector. Of course, you will have to determine where each wire goes on the monitor (not too hard if you're replacing the monitor also...just use the color coding in the old cable to match the ends). Matching the connectors to the old cable would be much easier if you can get a wiring diagram for your old system.
Having said that, running a new cable is not all that hard. It's just a lot of sliding under the coach securing the cable to the frame to avoid heat and road damage.
Good luck with your project.