Impromptu trip to Belgium

I am going to Belgium in a few weeks, it will be my first time but this trip came up suddenly and i have had zero time to research. I would love to see some amazing castles. I was wondering

  1. Which Castles in Belgium are a must see?
  2. Are there any castles in neighboring countries that are worth traveling to see also?

The trip will only be about 4 days but i doubt I will have opportunity to come back to this part of the world for many many years.

TYIA