4" is pretty small. I see that they have HD LCD monitors with inputs now for about 80 bucks.
But, with a 4" housing, you are going to have a small monitor. It would be nice if you could hack a monitor from an old video camera. Small and good on batteries.
I have the equinox rear screen system, but I'm not very happy because of the extra batteries and the weight. I have actually used a 9vdc transisitor battery for each dive.
If I was to do it, I would house the monitor, battery in a seperate housing that you can attach with short ball arm. Get a waterproof connector to feed it from your main housing. If you do the build, I'll follow the post! Happy desigining!