A couple of months ago, I was having trouble with our 92 Accord 4-cyl automatic (120,000 miles). The D4 light kept flashing and the transmission shifted erratically. There were no error codes.
From the research I had done, apparently it was a common problem with 92s. The consensus was that the TCU needed to be replaced. I bought a (used) one, and it was no better.
I disconnected the Throttle Sensor, the Lockup Control Solenoid Valves, and the Shift Control Solenoid Valves, and checked the voltage and resistance. Everything read within limits. I sprayed the connections with contact cleaner and re-assembled. I pulled the backup fuse for 30 seconds and tried it, and it was no better. For the heck of it, I stuck the original TCU back in, and the transmission shifted perfectly! No D4 light, either.
There must have been a bad connection in one of the plugs I disconnected, and the TCU I bought was apparently no good. The fix didn't show up until I re-installed the original (good) TCU.
It was a very frustrating exercise, but also very rewarding. I hope my experience helps someone else with this problem.
Tom

I'll almost bet the problem was with the solenoid connections.