I don't have a code table at home so I can't tell you what a code 4 means. Maybe someone else has a table. If you are only getting a single blink now you have erased the code and your light should be out.
Try it again when it comes back on. Only depress the button for 2-3 seconds. Six to eight seconds erases the code. I didn't mention that before as the cars with diagnostic modules and buttons have often worked slightly different that the federal cars that have to have an led added to retrieve codes (or use impulse counter).
Bosch Master, ASE Master, L1
33 years MB technician