You've verified by using a tool that no thermometer can do - your chicks' behavior. They are indicating they are comfortable without any outside heat source. Take away the heat lamp. It isn't doing anything for them.
By age four weeks, as long as chicks have been slowly exposed to cooler temps to stimulate feather growth, most will have a nice feather jacket zipped up to their little beaks. You can be confident they will not feel the cold.
Also the heat source that sits in the brooder is about the same heigh as the Percy.
As you can tell in this photo.