Can't speak specifically for this unit, but I have the current step up, the AVR4520 and I find it to be an excellent amp. In fact I prefer it to my old Onkyo TX-NR905, and that was widely regarded as being a class leader in its day.
I bought myself one for christmas. I upgraded from a 2105, so guess sort of a similar upgrade to what you're looking at.
I'm not really good with the "hifi lingo" but I'll just say I couldn't be happier. The difference in sound quality far exceeds my expectations. I now finally realise what my Monitor Audio silver series is supposed to sound like.
I don't think you'll be sorry if you go with the X4000.