In Belgium ATC are very helpful at providing clearances through Class D CTRs. They may specify a different altitude to your preference, but 4000 feet +/- 1000 feet should be possible.
In Netherlands I have found the opposite is true and it is easiest to stay low at 1000 feet. There are no obstacles to hit (but check the chart!) and you have a nice view of the scenery - mostly water features.
Try to visit Midden Zeeland - a very pleasant grass airfield near the coast.
Ostend Approach control the airspace around Ostend and Koksijde (that is from memory of a few years ago, so may have changed).