You will NOT make it to four weeks in such a small space. If you do, you will lose far more than you will "turn loose". And no, they can't be put in the coop without some sort of heating source at such a young age. Remember, they need heat until fully feathered. I say you need to get at least two more tanks for now and divide up your chicks to give them each more space. Of course when you divide them up, you then make separate flocks so expect some issues when you reintegrate them back together. By the way, how big is your coop? For 28 chickens you will need a BIG, huge even, coop.
The link below is for 10-12 chickens. So you'd need three that size.
Edited by JadedPhoenix - 3/8/16 at 7:01pm