Your systems have the minimum requirements for editing 4K. Minimum is not good or recommended and means you need to limit yourself heavily. High resolution (4K) is often usually the better choice (kind of depends on what you are doing with it). The 5.3K video is good for cropping and downscaling to 4K.
Regardless of the system strength, it is recommended to use Proxy and Preview Scaling features when working with higher than 1080p. For your systems, that is essential maybe even for HD editing. There is a tutorial on our Tutorials page about those features as well as Documentation - English pages here if you search by those feature names.
As for the crashing, there have been some crash bugs found in the latest release, and there is a beta you can try for the next release due by the end of the month