Four weeks old, sleeping *on top* of the heat source instead of under it, and making happy cheeps all day. That's a pretty clear message from a flock of feathery teenagers. They're telling you they're done with the brooder plate, even if one of them still looks a little underdressed. And honestly? Most of the time, you should listen to them.
Chicks pile on top of the plate for the same reason teenagers wear hoodies in July, they want the comfort, not the actual warmth. The plate still radiates a little residual heat, so it feels nice to loaf on, but they stopped huddling underneath it for a reason. If they were cold, they'd be stacked under that plate like sardines, cheeping in distress. Instead, they're using it as a perch and carrying on with their normal pecking-order squabbles. That's not a sign of discomfort. That's a sign of independence.
Now, about that slower-feathering chick, Pepper. You're right to keep an eye on her, but don't panic. A single late-bloomer in a mixed batch isn't unusual. If she's eating, drinking, and making normal chick noises, and you said she's flying at her sisters, so she's got energy, she's likely fine. The biggest risk is her being picked on while she's still fluffy, but you've already noted no blood and no real bullying. That's good. If you want to be extra cautious, keep the brooder plate on for another few days, but set it lower or prop it up so the heat is barely present. That way Pepper can still snuggle underneath if she needs to, while the rest of the gang gets their rooftop hangout.
Here's the practical takeaway: turn the brooder plate off tonight. Watch them for ten minutes after the lights go out. If they all pile on top of it still, you're golden. If Pepper starts shivering or cheeping in alarm, flip it back on low for a couple more nights. Chicks are better at telling you what they need than any chart or rule of thumb. They've already voted with their feet, and their fluffy butts. Trust the pile.