Generally, the higher the height setting the less vacuum you get.
Agree with Blue that that higher you raise the height, the larger the opening to the ground. However, when you cut your grass at height #4, you're also leaving behind grass at height #2; and the same goes for whatever height you cut at. So there's still a chance that you'll have sufficient vacuum at height #4 (or even #6).
But unless measured with scientific equipment, what one person considers acceptable vacuum will be deemed insufficient by another.
Ego Team: this is where the manufacturer should have demonstration videos showing real world performance trials or tests.