This exact same problem exists on the Note 3. (Same hardware) It's the Exynos, it's too weak. 1.9gHz is simply not enough, no matter how many cores it has. Cores do not stack. You need at least 2.2gHz individual for 4K.
The N9005 with the same hardware as the P905 handles 4K without issue (As does the P905).
The N900 has, from the very first day, been officially known to be unable to handle 4K. The P900 has identical hardware, and can't handle it either. The Exynos is simply not powerful enough.
Not to mention the Mali GPU is a graphical bottleneck.
Convert the video to a lower resolution. You can use Handbrake for it, it's open source.
This is one of the reasons I refuse to buy an Exynos device.