100 watt bulbs draw roughly 9 amps (give or take a little) on a 12v system. So 4 lights, times 9 amps equals 36 amps. I've burned up some of those 30 amp relays by running them too close to their rating for a long period of time, so yeah, I would run at LEAST two relays (18 amps per pair of lights).
The relay itself requires hardly any power to turn on, so pretty much any switch you like, you could use. Lighted, rocker, toggle, whatever, and you can daisy chain multiple relays off one switch.
I really, REALLY *REALLY* like pre-wired relay harness plugs. You can get them for pretty cheap online. I would recommend the double relay harness such as this one from parts-express:
Click me!. I have nothing to do with them, I just have one of their catalogs, and it had that double relay pre-wire harness. Then use the newer style heat shrink, water tight butt connectors and you've got almost a factory looking setup!
As for wire size, it all depends on distance, and amperage. From the battery to the relays, I would run a pretty decent size FUSED supply wire, prolly in the 8 gauge range, and then run either a 10 or 12 up to the lights.
If you need more fuse circuits, but dont want a pile of wires going to your battery, you can get a blueseas circuit block from west marine. Gives you like 6 additional circuits up to 30 amps each, with a total combined draw of about 100 amps. I have one of these in my superduty to run all my radios and strobe lights and stuff. Used a #4 wire to the blueseas box direct from the battery, then each piece of equipment gets its own fused circuit.