What do they do now? Do they put themselves to bed when it gets dark or huddle in the run? If they put themselves to bed in the coop I don't see that you have a problem. If they huddle in the run put them to bed in the coop after it gets dark. They should catch on. The question is when. I've had some learn for all to go in after a day or two. I've had broods of around 20 where the last few took three weeks to learn that, even with the others going in.The past few days it's been in the 70s and 80s during the day and they have been in the run, and in the henhouse at night with it in the mid-50s with access to the brooding plate. Starting tomorrow it will be in the low 60s during the day and low 50s/high 40s at night, and dropping.
Not all of mine walk up or down. Many of them fly. I find that what they want to do is much more important than what you think they can do. 3.5 week old chicks should be able to fly much better than many people think. Of course, if you have Silkies or other chicks that can't fly then that changes.
Or do you not give them that option? Do you put them in the coop before it gets dark and their instinct tells them to go to bed? In this case I'd wait until dark and see what they do. Base your actions on what you see instead of what you expect to see.
In any case I'd leave that brooder plate on. It won't hurt anything. They may not need it but if they do I'd want it available.
Good luck!