Did a 5 1/2 hour gig at a winery once with only two breaks,, last set was a solid two hours long, but nothing like 3 or 4 hours straight, that's crazy! I just lose focus without a break at least every 90 minutes or so.
The hardest gigging I've ever done was playing two big shows in the same night with two different bands. 6:20 to 8:40 with one 10-minute break in an outdoor pavilion for ~1200 people, then actually walked down the pedestrian mall here in our home town to a theater gig for ~400 people that started at 9:15. They did a couple acoustic numbers while ate a sandwich in the green room, I was on stage by 9:30 and did two set, the first an hour and 15 minutes, the second an hour and a 1/2. I was cramping up in my plucking hand by the end of the second set, unfortunately.