Putting in Coop Vs Brooder

I have been putting my girls outside during the day...closed coop and letting them run outside when temps warm up. They have been loving it. Temps have been high 60's to high 70's. Today temps are mid to low 50's and rainy. They hate their brooder box - have outgrown it!

They are 4 weeks old...I was going to put them in the closed coop but hubby says it is too cold. He wants to put a heat lamp out there. I am not hooked up for electric...think it is going to be way too much effort for a short period of time.

Do you think it is too cold for them out there today? Suppose to be like this for the next few days and I don't think they can handle being "cooped" up in their brooder box any longer than at nighttime!

 

Thanks!!