I lived in the Netherlands for a year, and had the opportunity to travel around Western Europe during the weekends.
Given your choice, I would definitely do the split trip between Belgium and Amsterdam, rather than only spending your time in Belgium
In my opinion, Brussels is quiet and there are far more things to do in Antwerp or Bruges (disclosure, I never been to Bruges). Even then though, 4 days is a lot of time to spend in Belgium. If you go, I recommend going on beer tours, look at the diamond districts, need to Google it, but there is the original place for Waffles in Brussels (as well as fry shacks, with the rich mayo topping), and finally the chocolate districts in Belgium and Antwerp
For Amsterdam, I rather spend 4 days there than Belgium. It can be a touristy place with all sorts of attractions you find in any tourist city (Chain restaurants, the wax museum), but there are some unique things. The Canal tour is a nice thing to do for your first time there. Also, the Ann Frank museum is a must in my opinion. Van Gogh museum, flower district, and the Heineken brewery are other places you may want to visit.
The red light district is much more touristy than I imagined, as well as the famous "coffee shops" if that's your thing. For food, I would suggest seeking out an authentic Indonesian restaurant, as the Dutch aren't known for their culinary offerings. However, Dutch things to try include Herring (whole fish swallowed whole with onions), fries and fried snacks from vending windows, and syrup waffles (freshly made are the best). You can also take a day renting a bike and exploring the city.
Anyways, that what I can remember from the time I lived there almost 10 years ago. You can always message me or reply back if you have questions about Amsterdam.
LChoy
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