How does vertical separation work above FL400?
For flights above FL410, 2000 feet of separation is used. Which makes all flight levels odd numbered, →410, ←430, →450, ←470, →490, ←510, ...
So for each direction as indicated above by arrows, it'll be 4000 feet.
The 2000 feet separation is because the higher up you go the less accurate an altimeter becomes, so it's for safe separation.
Even/odd is just an indication of direction in this case, which can be north/south or east/west, as explained here: Does ATC direct flights to fly eastbound at odd levels and westbound at even ones?

(Image source) Before RVSM came along, flights above FL290 were also separated by 2000 feet, compared to 1000 feet below FL290.

Between 1997 and 2005 RVSM was implemented in all of Europe, North Africa, Southeast Asia and North America, South America, and over the North Atlantic, South Atlantic, and Pacific Oceans.
[Between 2007 and 2011 China and Russia implemented RVSM.]

RVSM in essence is a reduction from 2000 to 1000 between FL290 and FL410 as long as the aircraft shows that its altimeters maintain their accuracy—among other things such as an operative autopilot.

Related:

Does ATC direct flights to fly eastbound at odd levels and westbound at even ones?
What is the maximum altitude of barometric altimeters?
Is it legal to fly in RVSM airspace with an inoperative autopilot?