For The Martian, they uprezzed the DI and then replaced live-action shots with 4K shots from the original footage. Any shots with VFX were left as 2K, though.
Even if a film has a 4K DI/master, the effects are likely still only in 2K. (Even big tentpole pictures that the studios know will be around for a while, like The Force Awakens, suffer from this.)
The "good" news is, even if the VFX or movie are only in 2K, the movie can still benefit from the extended color space on UHD Blu-rays, even if the picture isn't necessarily any higher resolution.