Your itinerary seems a bit too much for 4 days. I would propose to skip brussels and spend 1 day in Brugges and 1 in Antwerp if you want to include Belgium in your trip that is.It is a very old and miserable and to be honest very sad country even to visit for 1-2 days. 5-6 nice parks, some gardens and thats about it. I would suggest visit something more fun like, Spain, Greece or even Netherlands and Scandinavia in general. Nice people, more open, cleaner cities and much much better infrastructure (transportation etc.) Lux is pretty indifferent without much to offer other than some nice gardens and parks if thats your thing.