1994 Honda Accord D4 light blinking

I just had the engine of my 94 honda accord replaced. The guy who did the job took the transmission off and then put it back on. Now the car drives great except that the D4 indicator light is abnormal. Sometimes it blinks frequently, probably one time per second, but other times it is off. There doesn't seem to be a pattern. I have no knowledge about cars, but for some reason I checked the transmission fluid today. It is almost dry. I'm not sure how much transmission fluid the guy who did the engine work put, but he surely put some there. Can the low level of transmission fluid be causing the problem? Or is there a leakage which is causing the problem? A guy in Kildae said it could be a sensor problem. Which is more possible?

Big thanks!