How technical do you want your answer? I'm an electrical engineer, so I'll try and keep the techno babble to a minimum.
We'll assume 14v, one, for easy math, two, because 12v would imply your motor's not running. My Yj had the same alternator as your stock one, a 90 amp. Considering that MOST Jeeps have little in the way of power goodies - power windows, rear window defogger, electric radiator fans, etc, 90 amps can go a long way for accessories.
At 14v, a 100 watt light will draw ~7.1 amps.
You didn't mention the wattage, but you'll need to run each pair on a 30amp relay of it's own. All 4 on one relay would be pushing it.
Also, keep in mind that 90 amps is the max the alternator will put out at HIGH rpm. At idle, most alternators put out about 40% of their rated current, so a ballpark would be 36 amps.
If you idle a lot, and have the heater cranked full blast, headlights on high, with your fog lights on, your voltmeter will drop to 12v, and your alternator isn't going to keep up. So if you do a lot of idling or low RPM driving, consider a 160amp or larger alternator. If you keep your RPMs over 1200 most of the time, 4 lights should be fine on the electrical system completely stock.
I'm running dual Optima's and a 160amp alternator to power a 2,000 watt inverter so I can run a Lincoln Mig welder on the trail.