After comparing Amplitube 3 to 4 I have to say it seems that 3 is a bit livelier sounding. 3 is much louder when comparing identical setups and input/output levels. I have to turn up the master a fair bit on 4 to match levels and then when going back and forth there is a subtle difference in the way they sound with 4 seeming to my ears to be a bit darker sounding.
Agree, with regard to the sound if not the levels (I almost always use AT as an AU plugin to Logic/MainStage, and haven't really noticed a difference in levels). Among Apple's Amp Designer, AT3, and AT4, I'd say that Amp Designer is the most bass-heavy, AT3 the brightest, and AT4 sort of in the middle. I'm finding I'm adding a lot more treble and presence to the AT4 models, especially the new Marshall models, than I did with AT3.