K5000 has much more high quality waveforms than K4. K4 uses 16-bit sample, but the sampling frequency is limited in most waveforms.
K5000S/K5000R have only synthetic PCM waveforms. If you need acoustic emulation, go for K5000W.
I recommend K5000W, but I enjoy programming K4, too. K4 is simpler and sounds unique despite of lo-fi samples and filter click bug.
[url=http://deepsynthesis.net]Deep Synthesis Page: info and .mp3 sound examples
Recent updates: Roland D-110 / YAMAHA SU700 / KORG S3[/url]