In the coming weeks we plan to offer the option for PC players to launch the game on the 4.0 codebase to try it out.
Odyssey will still be an optional upgrade and necessary for on-foot content.
Players will still need to have the same version (Horizons or Odyssey) to instance together in game.
When the option to launch in 4.0 is made available to all PC players, instancing will be separate between 3.8 and 4.0 (as it has always been) and separate between Horizons and Odyssey on 4.0. There's no final state to confirm in situations like this which are always open to re-evaluation. If the situation changes, we'll make an announcement to that effect.
This is insane. We really need to convince FDev to at least allow instance sharing in space. I understand that on planet surface it may feel like a pay to win – but there's literally no reason not to allow Horizons players to interact with Odyssey players in space!!