As long as the heating pad can stay on continuously without turning itself off every hour or two, it's good. I prefer the one with the digital settings 1-6, but that's because my chicks are outside from the get-go and I like having the ability to "fine tune" it in the more fluctuating ambient temps out there. High, med, lows are just fine.
Tsk tsk, @aart - Ken would have chewed me a new, um, well - he'd have scolded me for using an older pad we had laying around the house. After enough time passes, the wires that conduct the heat through the pad can get a little brittle and might - the key word here is MIGHT - short out when it's suddenly left on 24/7 for weeks at a time. But if it worked for you, then we just won't tell Mr. Bossy Electrician!
@dyorto If they are moving out, do yourself and them a favor and turn the pad to 2. They'll suddenly be a whole new environment so they will most likely huddle, but it'll be more for security than for warmth. The world is a big scary place and like adult chickens, chicks don't like change. I tell people to put a big new something in the chicken coop with adults and watch. First they'll run like crazy to get away from it - or they'll freeze in place and stare at it, waiting for it to come and get them. But whichever one they do, they'll do it as close together as they can. Chicks in the confinement of a brooder don't have anyplace to run, so they huddle. And they are nicely feathered by now but that heat will still be welcome when they are suddenly in 30 degrees. They'll most likely pile on top of it...that's okay. The other thing I'd do if you can is raise the frame higher. By 4 weeks they are much taller, and might not even think to go under if it's too low to the ground. Remember that by 4 weeks they'd be sleeping more beside Mom - some moms even teach them to roost at this age - because they all won't fit under her anymore anyway. It's still nice for the first few days to give them the option.
Good luck, pour yourself a glass of wine, and keep us posted on how they do. Bet you go out tomorrow morning and they are all rarin' to go!
@lalaland My 15 were still using MHP at 3 weeks, and mostly on top by 4 weeks. Hope that helps.