On Hondas, a blinking D4 light almost always means there is a pressure switch in the transmission that has gone bad. You'll have either 2 or 3 on yours (can't remember on that model), but it will probably be either 2nd gear or 3rd gear pressure switch. They are very simple to replace as they just screw into the side of the transmission, and typically the part itself costs under $100 even from the dealer. Sorta related - If you haven't done it recently, get a fluid change while you're at it just for the car's health. Those transmissions get finicky after 200k
