I like almost all Spiderweb's RPGs, though for different reasons. Nethergate would slightly edge out the others if I had to vote for a "best".. I love the setting, the dual storylines, the system, the Phil Foglio pictures, etc etc. Actually, a slight digression: Jeff's mentioned doing an update of Nethergate that Carbonises it, increases the resolution, adds bits etc. I think that's a good idea, and will almost certainly buy it, but I just hope the system doesn't get overhauled for the worse. Though it was extremely minimalist compared to Exile III, I liked the explicability of various things - for example formulas like "hp = level * endurance" were right there on the character sheet. I don't think it worked quite so well in Avernum 1, but that's yet another digression.
As for the actual topic question: no Avernum 4's good, better than some of the Avernums, but it's no Geneforge/Nethergate/Exile III. It's simply too action-oriented to be the best.. although the action is rather good. Actual new 'tricks' and elements to fights were a nice surprise.