I own the 2 modules.
Ketron SD2 and SD4 have the exact same sound table.

I however discovered that the SD4 sound is more defined, maybe due to a better DA converters, it is more spaced. The SD2 sound is more compact but is more PUNCHY.

SD4 seems a little bit faster than SD4, in case of high midi demand. but SD2 seems more stable.

SD4 by USB seems to have a problem with the Window 7. I have to find the exact sysex and NRPN midi message who is creating the SD4 bug, and tell it to Ketron. I hope they will release a driver for Win7. For now I use it by MIDI not USB.

SD2 is smaler in size, but own RCA outputs.

I am still not sure if I prefer SD2 or SD4

Dan