To answer your original question, I'd say they *can* be left alone for four days, but it really depends on how guilty you'll feel if you come home and they haven't made it. Would you beat yourself up over it, wondering what happened or what had gone wrong? Or would you be able to accept any losses and go on? Me? I'm a wonderer and an analyzer. It would drive me crazy speculating over what had happened in my absence and I think I'd really regret it.
Secondly, heat lamps get, well, hot. Any time heat is involved, I wouldn't leave the house. One slip of that clamp light and you might find yourself coming home to a pile of rubble. Think it can't happen? Ask people who go to sleep with space heaters on. Heck, electric blankets have been known to start house fires.
I'd definitely do as others recommended. You've invested 21 days waiting for the little guys, so why not make a way to ensure that they're safe once they hatch? If a neighbor or friend can't help you out, maybe you could bring a portable setup to a vet's office, or a feed store. I'm sure there's someone around that wouldn't mind chick-sitting for a few days. Heck, I know I would!
Good luck, I wish I had the answer for your last question regarding the one who hatched. Have you read over the sticky thread at the beginning of this section? I remember it being packed with excellent information .. that I didn't find until it was too late. I have 17 days left before hatch day, so I'm planning to re-read it soon. Take care!