Sounds like you have decided what to do with the heat. It sounds like they are inside your house. I don't know what the temperature it is where they are but go by what you see. If they act like they don't need heat they probably don't. If they get cold you will start to hear plaintive peeps. That's a heart-rending peep where you recognize something is wrong. If you move them to another location, how warm is it there?
Some people are able to put chicks two weeks apart in age in the same brooder without any issues. Sometimes chicks can die if you try that. It sounds like that brooder is already getting small for your 4 chicks. They are going to keep growing, that brooder will just keep getting more crowded. The more crowded they are the more risk there is. I don't know how long you plan on keeping them inside before they go to the coop, that will have a lot of influence on whether that dog crate is big enough or not as they grow.