Generation 4 - LOVE that vacuum!
I'd have to say it is the FIRST one - the one I bought NEW (for around $700.00 when it first came out) - the G4. LOVED that machine! Lent it out to an older aunt of mine since the TECH DRIVE feature made it easier to use than her Kirby Legend II. But, as things happen, her daughter saw it and claimed it - so, I'll never see that one again.
Of course DISPOSABLE bags are a PLUS, yet, I'm REALLY loving a Kirby 514 (from 1954) that a co-worker gave me. (It cleaned and polished up a TREAT!) The nozzle width isn't as wide as the newer models, and that has actually been a plus. Down side is having to dump the bag onto newspaper (and rubbing the sides of the bag together to get the BIG chunks to rattle down); coupled with the fact that the hose has air loss, and IF I use the hose, little fibers that look like pubic hair are found in the emptor. So, don't use the attachments. Instead, I rely on the Electrolux AE for above floor stuff. (Although, DO wish that Electrolux had made a curved tube. Would be SO helpful for up high shelves and ceiling fan blades and the like.)
By the way, here's what I like about the older Kirby's - the floor buffing pad. It's like dust mopping and vacuuming at the same time. Neither the hard floor tool (with brushes) on the G4 OR the floor brush attachment on the Electrolux quite to the same job.
All the same, IF I could turn back time, would NOT have given the LUSH G4 away!