I owned one briefly, killer preamp section IMO. Of all if the hybrid heads I've tried, it had
the tubey-est sound, although I haven't tried the Aguilar DB-750 or 751.
I returned it because I couldn't get huge volume from it, but there's a link somewhere here where people very familiar with the 4-Pro give tips on how to set it's controls to get more than enough volume/headroom out of it. Had I found this thread, I'd probably still have one, and I'd be using it a bunch. I absolutely loved it's tone.