4th gear and torque converter both quit at same time, why?

Is the transmission fluid brown? Your car is 30 years old.

Car is 30 years old, but has been well maintained throughout its life. Fluid is not brown.

If you really want to know why ask the transmission shop that rebuilds your transmission what they find when they crack it open.

My guy doesn't use transmission shop. He's not a beginner, been working on his own vehicles for most of his 74 years (except the toddler stage lol). He is just looking for some direction please.

If anybody can give him just a little insight as to where to look for the problem, it'd be appreciated.

You will need a complete rebuild. I seriously doubt this is a case of replacing a part or two. If your guy can successfully rebuild automatic transmissions then my hat is off to him.

The bottom line is this - to find out what is wrong you need to take the transmission apart. Once you have it apart you need to do a Complete rebuild. This is the way it is. Find a top notch shop as most shops are really not very good. You could buy a rebuilt transmission and have your guy install it.