I have five Kirbys. One G4, and one Ultimate Diamond among them.
IMO the Ultimate Diamond IS the best of the modern Kirby line up. Only because of the "dual power" switch. Aesthetically they look the same, but with G4 there is extra work involved in vacuuming small mats, area rugs, etc. G4 will suck up the corners of the area rugs (not to the point of damaging them or anything), and move them. You have to bend over, rearrange the rug, and keep going. Only other option, is to raise the nozzle (to lessen the suction), in which case the brush doesn't come in the contact with the rug, and the pick up suffers. Also,...if the rug is not agitated, what's the point of vacuuming, especially if we are talking about pet hair (pet hair can seldom be picked up with suction only)?
With Ultimate Diamond, you flip the switch to "delicate", and keep the nozzle lowered, so the brush is still agitating, but the suction is much less (but still adequate to suck in the pet hair, and the rest). Rugs stay put, no extra bending, rearranging, etc. I like that switch very much.
This switch is in lieu of the "suction relief vent" that was on the older Kirbys (which I also have) that I also use weekly (I use all of my machines, knowing their abilities and limitations). Kirbys have a LOT of suction, but sometimes the level of suction needs to be regulated. All of the G series (except Diamond), and all models after it, don't have the ability to regulate the suction (in the upright mode). Of course there is a handle for the wand (when used as a canister with hose attached) that has the relief valve on it, but no possibility of suction relief in the upright configuration.
Why was this switch omitted from Sentria 1 and 2, and from the new Avalir, is a mystery to me. Models that came after the Ultimate Diamond, are more expensive, and don't have ANY advantage over it, except cosmetic changes.
I like my G4, but when I have to go over smaller (and thinner) area rugs, or mats, I use the Diamond.
Hope this helps.