Can a 4 inch door jam be installed properly in a 6 in wall?

It will take a bit of extra work, but it will turn out great. I suggest looking at utube tutorials, that way you can see exactly how to do this properly. So happy you are up to the task! 👍

You can order doors that fit 6" walls. I was remodeling a home built in 1950. I never thought of the thickness of the outside walls. I special order 3 doors and when I installed them they were to narrow to fit the opening. I didn't want to return them. I installed the doors to the outside walls and it left uncovered area on the inside wall. I used 1x4 and sawed them the right thickness and covered the gap. It turned out looking very nice. The bad problem is the door knob tongue hits on the wood and wore a groove. I found some brass and was able to form it to cover the groove. Now it is fine. Next time I would install to fit the inside of the door frame.

You sure can. Instead of using one solid 6" jamb (which are hard to come by) your using a 4" jamb and adding 2" or so wood to fill in the balance. It definitely will work.

oh yes it will. You'll get a wierd seam in the flat part of the door casing.

Totally!

Thanks