Four 4k simultaneous playbacks from new Macbook Pro? Is the new 15" Macbook Pro 2018 capable of playing four 4k videos on four 4k displays simultaneously?
Yes.

Although you may be pushing the system to its limits, it's technically possible to drive 4 simultaneous 4K displays using the new 2018 15" MacBook Pro.

From the Video Support section of MacBook Pro Technical Specifications page: