I can feel the extra load on the engine switching from 2High into 4Auto.

Very slight but you should notice it if you set the cruise at (say) 35mph on a flat road and switch between the two modes. 2H gives better fuel economy due to less strain but that probably takes laboratory instruments to measure.

I run 2H all the time other than in the rain where it's easy enough to rotate the switch to 4Auto. I rarely use 4High except on very loose surfaces and only use 4Low to make sure it still works and occasionally when hooking up my camper as the truck crawls in that setting and I can hit the coupler easier.

My Explorer only has 4Auto, 4H, and 4L and has been running in 4Auto for 120,000 miles.

-- Chuck