They get off the eggs about once a day to eat and drink, and then return to the nest.
I have a much larger flock, and the problem that I experience is that when the hen gets up to eat, another hen takes the nest temporarily to lay an egg, while original hen doesn't go back to the right box.
This means that the eggs get cold, and extra eggs get laid from a different day, and eventually the whole thing fails.
With just four hens, it may be manageable though.
I'd let both hens continue sitting if both are broody. Sometimes a hen will give up--and then if it does, you will want to immediately transfer the eggs to the hen that is still sitting.
What breed are the other chickens and the rooster? I have always left the chicks in with the general flock and have not had problems. I stay with chickens that tend to be more gentle.
Once the chicks hatch you'll want to make sure they have 24/7 access to chick feed and small waterers that they can reach. I use plastic fencing intended for human babies or dogs (the superyard play yard) to make separate areas in the coop for the young chicks. The problem is less about aggression and more that the adults eat all the chick food instead of their own food and then the chicks have none!!! Also, the chicks (and the mother hen!) tend to dump over the feeders quite often, so I ended up screwing plastic chick feeders onto a piece of plywood so that they couldn't tip it over. Mommy hen is wonderful, but she's so interested in teaching the chicks how to scratch that she dumps EVERYTHING over!
For water, I prefer a wall-mounted waterer that I mount about half a foot or more too high for the chicks to reach. Then I arrange bricks and cinder blocks for the chicks to climb to get to the water. Keeps the water clean and un-dumped (see previous paragraph) and makes a fun obstacle course, too.