Personally I'd have no problem with four week old chicks being off heat and outdoors full time in those temperatures, so long as they were able to stay out of the wind and rain and there were more than a few of them to snuggle together and stay warm. Other people might be a bit more cautious but they should definitely be fine to be out full-time by six weeks old if you really want to play it safe.
I would make sure they have a separate space at first that only they can access, to get away from the older chickens once you begin integrating them. Putting them out while they're still young helps here because it's easier to create a "safe room" with a little chick-sized door if they're still much smaller than the older birds.
Edit - personally I also wouldn't keep them inside until they're fully feathered. Even short trips outside will be good for them, if you can manage that with your current set-up.