um, careful with running the lines out the back like that. There are probably cooling lines around all 4 sides. You hit one of those, you're in for an expensive repair. Conventional set up is to use flexible PVC and go straight out the top. But if you think you've got it under control, go for it.
I think I would go with two standard two tap conversion kits, I don't see how the premium is worth an addition $125 each. But with two kits, you'd also need two seperate co2 tanks. They do make secondary pressure guages that all run off of one primary guage. You would add THIS onto one primary regulator and maybe use a 10lb co2 tank since you're running 4 taps.
http://www.micromatic.com/draft-keg-...-pid-3033.html
If you called micromatic, they may be able to set you up with everything you need in a 'kit' price with the regulators, faucets, etc. I've heard that they're quite flexible.