[Walt Fricke]

If you can get 2d and reverse, it is not a shift coupler or bad cup bushing up front. Those gears require pulling the shift coupler forward about the same distance.

If the shift lever, when 3d is engaged, is in the same forward position as it is in 1st and 3d, then it doesn't seem likely that the Y shaped yoke/shift fork inside the transmission has come loose and slid forward on its shaft inside the transmission, away from the 4th gear. But perhaps somehow it slides when pushed toward 4th, but slides back and then catches when moved to 3d?

Removing the shift fork plate (4 bolts, drain oil first) underneath will let you look in and see some of all this. You can push against the slotted forged pieces which move the 1/2 and 3/4 shift rods. Maybe you can see the brass shift yoke/fork, which will be to the rear of the other one serving 1/2, which is toward the front of the transmission/car.

I'm seeing a transmission disassembly in your future, as others do. Doesn't hurt to look first, though, as you'll be draining the tranny oil when you start disassembling it on your engine stand anyway.

I don't quite know how, if all these other tests show things there are right, you could pull all the way back on the shift knob for 4th gear and not have something happen. Nasty grinding sounds due to failed synchronizer pieces or worn engagement teeth. Though I have seen the "dog" engagement teeth, the ones which engage the teeth on the slider, worn down to less than nubs.

This should leave quite a bit of metal shavings in the transmission oil - more I'd think than the magnet in the plug can hold?