Well it looks like I simply don't have fourth gear. Changed the fluid, looked just fine (ruby red in color, no debris in pan, fluid at a good level). Changed the filter again for the heck of it. Manually shifted through the gears as suggested. No 4th. Even fiddled with the bowden cable, no luck.

Only thing I changed was the green modulator on the left side on the used trans.

The guy parting out the car had all the paper work for service history, 150k, serviced every 30k or every two years.

What could have I missed? Valve body? Broken spring?

I'm stumpted on this one.