I personally prefer to have the CO2 outside but its really just that - preference. If you keep it inside, you lose a little space that could be used for kegs, but you save the space and aesthetics from having it outside. Outside gives you more room, but can be more susceptible to damage and can be ugly with the gas hose and the tank.
If you run it outside, just run one line from the regulator to the inside, and have that attached to a manifold that is mounted inside and distributes to the different kegs.
4 taps for a fridge is just fine assuming you can put all 4 kegs inside, which shouldn't be a problem.