As the Honda Owner’s Manual tells you, a blinking D4 light indicates an electronic problem relating to the transmission. And, it also states that failure to fix the electronic problem promptly can lead to more expensive mechanical problems. In other words, don’t continue to drive it. (If you don’t have an Owner’s Manual, this is a perfect example of why you need to get one and to read it!)
In this case, the fix could be as simple as replacing a Vehicle Speed Sensor (VSS), or it could be as expensive as replacing the Transmission Control Module. Only examination by a competent mechanic can tell you for sure. I suggest that you take the car to an independent transmission shop for an accurate diagnosis. Whatever you do, DO NOT go to AAMCO, Lee Myles, Cottman, Mr. Transmission, or any other chain operation unless you want their standard diagnosis of, “You need a new/overhauled transmission”.
When the car is at the trans shop for diagnosis, I suggest that you also have them change the fluid and trans filter if those procedures have not been done in the last 3 yrs/30k miles.