I can always take 2-3 qts, and i stop there because it feels to crampy & painful.
To be able to take 4 qts, I find that first I need to have one (2-3 qts) just to clean out, and then I need to let the colon rest & relax or a while -- maybe even take a nap for an hour or two, or do something else. Then after the colon has relaxed, I can take up to 4 qts if the temp is about body-temp, and I do it slowly. While I may feel fairly full at 2.5 qts, and begin to get some cramping, I find that if I stop for a bit and then continue, I can take a full 4 qts even tho I felt quite full for a while. Often I'm surprised that the last few cups seem to go in more easily without pain or cramping than the prior qt, even tho I feel quite full.. At 4 qts I feel like a very inflated balloon.
Surprisingly, I find that a very short plug type nozzle works better than larger or any inflatable, because it seems to stimulate colonic contractions less. I don't believe I've ever felt comfortable enough to take 4 qts using any large or inflatable nozzle; the pressure seems worse with them.
In general, I always STOP if I feel any pain or cramping. While it's interesting to feel a full 4 qts going in, I don't think it's worth it to hurt yourself to achieve that goal.