Yes and no.
If they choose to stick with the h.264 codec, then yes, 4k movies will be around 200gb in size, but they'll most likely make the switchover to h.265 which is twice as efficient as h.264, meaning 4k movies would be around 100gb in size.
Edit - Apparently, triple layer BDXL standard blu-ray discs have a capacity of 100gb, which would line up well with the expected file size of 4k encoded movies.
https://en.wikipedia.org/wiki/Blu-ray_Disc#BDXL
It should allow for 4k encoded movies to be burned onto a single disc.