score:4
Slight correction:
However, we heard that sometimes the police at the border used to make a fuss and you end up paying for some rules they make up.
Nowadays, the Serbs are really the most lax and nonchalant border officers I've ever dealt with. Chances are they won't even ask for car-related papers for a Hungarian-plated car, but simply check your passport or identiteitskaart and send you on your way.
If you use a passport, your main concern should be getting an entry stamp, which they're often too lazy to do. An exit stamp isn't needed, however.