At 3 and 4 weeks old they would still be with their mama in nature. Chicks do not bed down in straw like rabbits, for example. So I believe they need extra heat, if your temps are not around 20-25C at night, for a couple of more weeks. And chicks are meant to get heat from above, that is why heat lamps are used for chicks. But they will need extra heat only for a few more weeks until fully feathered. In your case I would use hot water bottles, preferable arranged so they can creep under the bottles in a small space. Think mother hen. Perhaps you can hang the bottles in a bag in a corner. Of course a mother hen does not cool off during the night, so the chicks must be able too move away and towards the heat source according to temperature. And check very often in the beginning to see how it works.
Please any opinions welcome, I am collecting tomoz and really need to know what set up is best, they are off the heat lamp, and the person i am getting them off has them in a brooder in the garage, so please do I keep them in or put them out? If I keep them in do they need food and water during the night or just during the day in the run?
If you have a heat lamp in the coop, go ahead and put them out. As long as they are locked in at night so they cant do anything stupid like getting stuck outside they will be fine. My 2 week old leghorns are in the outside coop w/ their heat lamp and are doing fine. When it is nice I open the door for them to venture out but It has been hitting freezing over night so they get locked in about 5:00 pm. hope this helps.
I also have food & water available 24/7. With the lamp they don't just sleep at night like they will when older.