4 wheels which can run in any direction give you more ways to push or pull the cases and a better behavior in tight spaces, curves…
4 wheels on rigid axis are good for a large suitcase (they still give you more ways to move it than 2 wheels), but not much of an advantage for a small one.
The only suitcase I have which is discreet is an inheritance; it’s navy but does have some red stripes which make it easy to identify on the luggage wheel. The ones I’ve bought myself are recognizable from the other end of the airport luggage room.