SixFinger,
May be missing something, but the rebuild from 3 to 4 was semi automatic on my system. Previous stomps and racks, cabs from Amp 3 were automatically there in ver 4. Took awhile after opening the preset browser to get all the previous presets recognized. Don't have any custom presets from 3 in the program, so can't testify if they come through. Should work however.
As far as I can tell, everything from Amp 3 (stomps, cabs, racks, presets) translate to ver 4. Only question I ain't resolved yet is if ver 4 is some cosmic upgrade into "hyper realism", or after all the hype, a minor tweak of ver 3.
Still searching for the silver bullet they claim is there. Doesn't mean it is not there just because I haven't found it yet.
John