Hello, in a multiplayer game, does a Orbital Resupply Station only effect it's owner or both players of the Dark Side ?

Orbital Resupply Station

Reduce the cost to play each [imperial Navy] Vehicle unit with a printed cost of 4 or higher by 1.

By example, player A of DS has one of them in play, and player B has a Devastator (Navy vehicle costing more than 4) in his hand. Does the Devastator cost 5 or 6 resources ?

It affects both players by my reading of it =D

In your example that means it costs 5

Yep. Constants apply to everyone.

Unless mentionned otherwise, which it is not.