They can go outside at any time you can maintain the temp. in the coop to the level needed based on their age. At four weeks old, that would 75 to 80 degrees, perhaps a little cooler.
If I lived in sunny California (like new2chooks) my chicks would be spending every warm, non-breezy day outside and only going under their heat lamp at night; assuming it was chilly enough for their heat lamp at night even.
I raised my chooks outside. They started out in their brooder in a draft free shed for the first week or so, but by week two the shed was too warm during the day, since I couldn't leave the doors open to predators.
At two weeks they were living in their brooder, with their heat lamp and a draft guard on our screened porch. By four weeks they were without a heatlamp during the day; by 5 and a half weeks they were without it full time.
I started taking them outside on nice days at two weeks. They loved running around and it prepared them well for their future.
This is just an account of my experience and it worked so well for me, it's how I will always brood chicks.