4k preview is tricky. However this may be a RAM allocation issue. Check your preferences and make sure that your RAM isn't set to 200 MB or something!!!
I believe these software's cache the videos in the RAM or something. I've had issues like this in the past using After Effects and this technique works like a charm.
Just mess around with hardware acceleration, GPU settings, cache, and RAM in your preferences.
If none of this works. The bottleneck is either the software, or SSD. Are you using SATA 3.0 cables? What are your SSD speeds?
The bottleneck in this situation is almost always the drive.
Good luck!