I can't give a comparison because I've never played a G IV, but have owned a couple of G IIIs since '95 (yes, they're post Fender!). I've been very satisfied with mine, and feel it presents a very big sound. Plenty of bass, good mids, and more highs than I can use, with a ton of power handling.
The reason nobody's fawning over SWR cabs here seems largely due to the "flavor of the month" syndrome that is so prevalent on these forums. Also, many say quality was compromised when the "Evil Giant Fender" bought the once beloved small company. I can attest, though, to the build quality of my '95 model cabs, as Hurricane Katrina stripped one bare. I got to see the cab construction with all components and covering removed. VERY GOOD!!
From what I've read, the main difference is the use of Eminence drivers in the the IVs rather than PAS in the IIIs. I was told by my reconer in '95, that there wasn't a better 10" for bass guitar, so I proceeded with a full recone job on the PAS woofers.
Eminence makes the speakers in the majority of cabs these days, and I feel that SWR is using a quality product in the current line. The biggest negative for me is the weight. It's getting harder for me to load my SUV with mine, so I leave it in the band trailer and let the road crew deal with it while I search for a lighter, yet ballsy cab that I can substitute for it.
Just my two cents!