I don’t believe in magic numbers, either space or temperatures. There are just so many different variables in all of this that what works for one person might not work for another.
I understand you are not going to have electricity in the coop. I do and I’ll tell you what I do to maybe give you some ideas for your garage. My 3’ x 6’ brooder is permanently built into the coop. I put chicks in there straight from the incubator or straight from the post office, whether the temperature is below freezing or in the heat of summer. In winter I wrap it in plastic and use heat lamps to keep one end toasty, but there are times the far end has frost in it when it is really cold. There are plenty of different ways to provide heat but the goal is to have one area warm enough on the coldest days and one area cool enough on the hottest days. The chicks are really good at self-regulating themselves when given the option. In summer I do not wrap it in plastic and use a much lower wattage heat lamp on one end.
As far as size goes, your rabbit cage sounds great. I used to have a 3’ x 5’ brooder. I kept 28 chicks (mostly female) in that until they were five weeks old. That’s about a half a square foot per chick. They were getting a little crowded but I did not have any problems. Another time I only had 21 chicks in that until they were five weeks old but they were mostly males. They were starting to get kind of crowded. The more chicks you have the less square feet per chick you need because they have more empty room to explore. Sex, age, and number of chicks are just some of the variables that affect how much room you need. With four chicks you will have plenty of room for food, water, and room for them to play for over five weeks of age.
As far as temperature, one record setting hot summer I turned the daytime heat off at two days and the overnight heat off at 5 days. Their body language told me they did not need heat and they did not. They were fine. In the winter I often keep the heat on until five weeks of age. I’ve gone a little longer when it is well below freezing whether I need to or not. I have taken chicks from that brooder and put them in my unheated grow-out coop at five weeks with the overnight lows in the low 40’s Fahrenheit. I’ve had chicks go through overnight lows of the mid 20’s with no heat before they were six weeks old. These chicks were acclimated to cold temperatures by playing in the cold end of my brooder. The grow-out coop had good ventilation up high but good draft protection where they were. So some variables of when they can go out is how well they are acclimated and how well your coop is built. Ventilation is good but a cold wind hitting them is not good.
If you set your brooder up in your house it probably doesn’t take much to provide enough heat. Whether you use heat lamps, incandescent bulbs, heating pads, heat emitters, or something else, try to provide heat in one area but let the far end cool off as much as you can. Putting some kind of barrier to provide “shade” from the heat might help. Overheating them is as bad or worse than too little heat. Putting a thermometer in there to give you an idea of temperature may help your self-confidence and ease your stress, but watching the chicks will work a lot better. If they are as far from the heat as they can get it is to warm. If they are huddling as close to the heat as they can get they are cold. If they play in different parts they are great.
If your garage is cooler or you open and close the garage door you may have a bit more of a challenge by putting it out there. In your house with stable temperatures it is pretty easy to stabilize temperatures in the brooder. If the temperature is going up and down you cannot stabilize temperatures very well. That’s where keeping one end warm enough and the other end cool enough is really handy. If you do that and let them self-regulate that takes all the pressure off of you. And they get acclimated to cooler temperatures.
It sounds like you are set up for success and have some options. Good luck!
