The GNX4 falls under the previous generation of modelers by Digitech eg with GNX2 and GNX3. It is manufactured to be the 'bells and whistles' workstation of that series, thus the drum machine, recording functions etc. It uses pretty much the same technology as the GNX2 and 3, just more powerful.
The GNX3000, on the other hand, is the latest version of Digitech modelers, with newer technology. It lacks some of the features of the GNX4 though, eg. simpler drum machine and recording functions. Think of it as the newer version of GNX2, features wise.
Sound-wise, the GNX4 is kinda similar to the GNX2, which I had for 3 years or so. It sounds pretty good, but some of the higher gain models and modulation effects can sound pretty fizzy or erm... 'digital'. If you set it up properly and be more subtle with the settings though, it sounds perfectly usable in a band context. The sound can't really cut through if there's another guitarist though, but that might have to do with my settings. Works well for recording.
No experience with the GNX3000, but reviews I've seen have been pretty effervescent in their praise. My favourite guitar mag for reviews. Guitarist, gave it a perfect 5 in all areas, something I've seen only around 4-5 times in the past 5 years or so reading the mag. Hmmm some quotes...
"Some of the more high end features of the GNX4 have been left off, as has the built-in multitracker, which makes the whole thing seem more concise and focused. Add to this the newly improved amp models, and you have a unit that just seems to do a great job with the minimum of fuss and complexity."
Sounds good. Go Swee Lee and try la.