I've got the same problem on a 92 legend. The D4 flashes after
about 3 minutes of driving and does not stop until shut off.
No engine light, no problems with the way it drives or shifts.
I bought a HELMS manual and they show how to apply a wire to short across the ECM and the D4 light will flash a "code", telling you what the problem is.
I'm tracking down that now.
It happened to my 91 Legend. It meant thet my vehicle speed sensor was no good.