At four weeks, chicks are pretty much finished with needing any extra heat. Just make sure they aren't subjected to cold drafts. Letting them get used to outdoor temps by exposing them to day trips out to the run would be a prudent first step, though. Gradual change, rather than abrupt change, is always good practice when it comes to chickens of any age.
The panic room method also makes it much easier on the chicks when integrating with the adult flock. It involves a safe space in the run with chick-size openings where only the chicks can fit and the adults aren't able to follow. Take a look at my article linked below this post on outdoor brooding to see how this works.
This safe refuge allows for chicks as young as two or three weeks to safely mingle with adult chickens, and all who've used it report their chicks integrated safely and easily. The most critical factor in integration of much smaller chicks is having their food and water in a safe place so they aren't chased away by older birds. It can deprive them of both needed nourishment and self confidence otherwise.
If you rig a panic room enclosure, after a week of observing the adult flock, chicks can then be allowed to mingle. This is ideally the precursor to moving into the coop with the adults since it establishes a "game plan" for the chicks for daytime. They will know where safety is and hurry to their panic room when morning comes.
