Transmission Options

Every time I use a 2004R or a 700R4 in one of my cars I install a non-lock up TC. I also use all the "good" parts like hardened drums, etc. If you use a non-lock converter you don't need any switches and wires to lock and unlock the converter. I cars shifts into OD at around 45 MPH and stay in OD unless you accelerate then it will downshift to third gear or second (passing gears) depending on your speed. Once you let up on the throttle it goes back into OD as long as you're going over 45 MPH.
To answer your question directly, YES...it stays in OD to top speed unless you punch it and make it downshift in which case the transmission "thinks" you're passing someone on the highway just like any other transmission downshifts. As soon as you let up on the throttle it goes back into OD.
You are correct, 4th gear wasn't designed for pulling heavy loads. Even modern cars (2016) have buttons (switches) so you can turn off the OD function when pulling a trailer or going up steep inclines.

Treblig