I worked at Chrysler’s Desert Proving Grounds during the time the switch was being considered to use ATF+4 for all vehicle power steering systems, not just the Jeeps. On a number of our power train test vehicles (all models) we ran the steering systems with a 50/50 mix of the std. Mopar power steering fluid and the ATF+4 to test for later compatibility issues in the field. There were no issues what so ever in these vehicles using the 50/50 mix for the full duration of the power train test.
IMO the ATF+4 is a better choice in all respects.
That being said, it sure wouldn’t hurt to get all the old fluid out if you do change over but don’t take a chance on running your pump dry emptying the system. A little left over old fluid in the system won’t hurt a thing.