These threads, and the Archives, are stiff with references to the older 9140-4. Me, I recently got one (older brown model, under $320 on eBay), and have used it twice. The second time was yesterday on the Skagit, trying it side-by-side with a demo 7141-4. The newer rod, lined with a Mastery 8/9 spey line, was fine: a lively, medium-fast dandy for smaller flies (I fished #6's). The older rod seemed slower and less powerful by comparison , lined with a custom-spliced DT10, but also user -friendly at shorter distances (I unveiled it on the Vedder R. last week).
Then on a hunch, I switched the lines. The 7141 didn't feel overloaded with the 10, just as flat as a telemarketer at the end of a double shift. The 9140, with the 8/9 long-belly, seemed a lot livelier, much more like the 7141. (This is the first time I've had a spey rod perform well with a line lighter than its rating.) Both rods easily cast 80-85 feet.
I also tried an XLT 8/9 on the 9140 last week. It seemed fine, but I was casting about 50 feet on the shrunken Chehalis River.
I'd be happy with either rod on a summer steelhead river. I'd want to build a 7141 blank with trimmer, lighter fittings than its overly-plump stock form. Sorry that I can't offer a comparison with the 9141-4. I haven't tried one, but no doubt others have.