Mastering at 4k does cost a lot more in rendering time and all that - after all, if you're rendering 4 times as many pixels, it's going to take roughly four times as long, and considering the potential market, I don't see how redoing the VFX will be a good return on the money spent. It's not just a case of re rendering the scenes, but someone has to make sure they come out right. Because on a 2d video, shortcuts are taken that you cannot see, and those can easily break down if you just blindly reapply at a higher resolution.
In the case of the Martian, the VFX scenes are basically the scenes you want to see in 4K - all the outside scenes. Heck, the visors on the space suits are entirely VFX added in post, including reflections off the sun visor and off the visor itself. (There's a video showing the VFX applied on YouTube by the VFX company).
In the case of movies, the home video release is a minor source of additional income, so I think it's unlikely they will redo the VFX. Likely what happens is like the IMAX releases - the scenes shot in IMAX will be clearer, so the scenes shot in 4k will be clearer, while the 2k bits will be merely upscaled.
And given dropping physical disc sales, I doubt much investment will be made for UHD Blu-rays specifically. Heck, they probably will be quite expensive limiting the market further.