Yes, or you could bring them in for the night - here are the temps you need:
First week of life 90-95 degrees, and decrease by 5 degrees per week until fully feathered at 6 weeks or therabouts (in winter I go a bit longer).
So your 4 week olds need access to 75-80 degree light bulb warmth, with room to get away from the heat as always. You can use lower wattage bulbs if your ambient temperature requires it.