The GNX4 was released around the same time as the GNX3000, and after comparing the two for a while, I got the GNX3000.
This link has a good breakdown of the features of each, and I think the focus of the GNX4 was more from a DAW perspective, where the GNX300 was more "effects controller." I have not actually used the GNX3000 for DAW work or re-amping, but simply to jack it into a clean amp when the mood strikes. For that purpose it has gotten its money's worth and then some. I did not realize until I found that link that the amp modeling is 3:1 different between the two models though, and I cannot speak to the GNX4 at all, but personally would go with the GNX3000 again (for my use). For live work, many lauded the GNX4 due to its internal looping feature (and many considered the GNX3000 a bad alternative for this); so much depends on your intended usage.
The pawn shop part makes me very leery, as you would definitely want to give that a workout before you bought it. I am not sure if they would allow such, but I would definitely want to test not only the amp out, but headphone out with my own guitar (if they have amps there) before I would even consider a purchase. Physically things may look okay, but when electronics are concerned the proof is in the pudding.