4 week old chicks outside in AZ?

Do you all think it would be safe to start integrating my two 4-week old chicks to the two big girls in my coop? Highs during day are about 90, and lows are about 65. At this point, I do not have heat in there for night time, but I am planning on wiring in a lamp for heat for winter. ( I am planning on housing the chicks separately for a few days until the big girls get used to the sight/smell of the small ones)

You are taking a prudent path in keeping the 4-week olds separate but in view of the older chickens for a bit until the older hens become accustomed to them. But chickens do not "smell" each other. They instead learn the details of each chicken, especially the faces, and will recognize their flock members. They recognize human family and friends, as well.

It will be very simple integrating them if you keep a safe pen or cage for them to go into that the larger hens can't follow. I call it a panic room. Food and water are inside so the chicks don't need to compete for essentials, and they can escape the pecking order when it gets to be too much stress.

I have eight-week olds who enjoy lounging in their safe pen away from the hectic pace of the rest of the run, even though they're entirely capable of dealing with it at this point. In another couple weeks, they will no longer fit through the 5 x 7 inch openings and will have to give up their safe haven then.

As far as your heat issues, you really do not have any. I doubt you will even need to worry about heat of any sort in your coop in winter, either. Chickens are quite comfortable sleeping in temperatures down to freezing at night, and will adapt to even lower than that.

I've had chicks outside since about 3 weeks old. It's been between 40 to 50 degrees overnight. They've been fine. I should add that they feathered in very,very quickly. They were completely feathered by the time they were 4 weeks old.
Your chicks will likely be fine at your temps. even without being completely feathered. They will need some time to grow before they are big enough to handle pecking order scuffles.