Is there no automatic transmission guru reading this?

This Brickboard has been ultra-helpful to me in past years. It has proven to be one of the very best things I've encountered on the internet. People sharing their specialized information & real world experience. Beautiful.

Although I am feeling frustrated by the lack of response to my basic transmission questions, I'm going to try it again. I often skip over questions that have been covered thoroughly in the archives + FAQ's... so, maybe I'm doing this wrong? Or maybe I've missed something in the archives.? Maybe there simply is no automatic transmission wiz reading this?

I think that the fluid is cut-off from 4th gear for some internal hydraulic (or other) reason that I have no experience with...

Ignoring the OD relay issue, and solenoid fixes for a moment ---
this transmission hasn't exhibited other telltale signs of a 'worn-out' automatic. But there is no 4th gear; I have some fundamental questions as a result ---

QUESTION: What internal problems could cause a AW71 not to shift into the overdrive 4th gear?

What safe & sane procedures might I try
to restore the engage/disengage of 4th gear?

thanks,
Sincerely, Bryan Crawford