Wow, I just ordered a MAX454 chip, and going to build multiplexer for my cameras. Been using a relay/555 timer to switch between carport and back yard on a small LCD monitor, and a VCR to switch between 4 cameras on the main monitor with remote. I think the 4-plex view would be a tough to build idea.
Have you consider a video input card and PC, you could configure the software anyway you want, change easily and slightly cheaper than a dedicated multiplexer (but still more then I'm willing to spend).
The MAX chip is $9.95 and 4 channels and video amplifier, channel selection is digital using 3 bits. I'll be using an ATtiny2313 to control switching. I once tried a 4066 bi-directional switch, but didn't get good results.