I can't say I'd recommend driving in either city, but it you're writing from Wiesbaden, you probably have a car anyway...
4 days is a little long for Bruges. There aren't that many museums or other sites to tour. Although the old town is a delight to explore, both on foot and bicycle, I can't imagine you could fill four days doing just this.
Brussels, of course, is a big city. Although not nearly as pretty as Bruges, I find it overall more interesting. It has far more museums, better restaurants (for the most part), and it contains a fascinating mixture of architecture from many different periods. But it isn't nearly as walkable as Bruges, and it takes some exploring and investigation to really open it. It seems to me that most people who find Brussels uninteresting simply get off at the central station, take a look at Mannekin Pis and the Grand Place and assume there's nothing more.
Another option- take a daytrip to one of the beaches or to Ghent or Antwerp from Bruges.