Max UHD disc speed is 128Mb/s / 16MB/s. Hard drives read at up to ~150-200MB/s, so far from being a limiting factor. Even the slowest 5400 RPM hard drive is still fast enough to play any 4K video.
Even SMR hard drives which do slow down as you full up the drive and/or rewrite files, are fine as the the read speed is the same as CMR drives.
Your issues with crashes and stutters is caused by something else other than the hard drive.
Note that this is for video playback. If you're writing or editing video, then an SSD makes a difference.