This jack leg (me} has never heard cornwall 3's or 4's. That said, I was a Klipsch dealer from 1989-1993 and was open from 1885-1993. Also sold B&W, Vandersteen, Eminent Technology, Mordaunt Short, Spendor, and other speakers. I recall taking a pair of Cornwall 1's in on trade, dont recall on what, but DO recall being astonished on just how good the CW'S sounded, especially with low wattage tube gear. The cornwalls were a discontinued item at that time, and I seriously wondered why. They certainly didnt have the highest quality drivers, crossover networks, etc but they did make you wake up and go WTF? when you threw on some audiophile material. At the time we had Khorns, chorus's, forte's, quartets, kg4s, kg2s, and I dont know what else, and I was impressed morseso by these 70's cornwall trade ins. I would have kept them for myself at the time if I could have afforded to. Circa 1990. Other trade ins I took home and later sincerely regret parting with were Apogee Duetta Signatures, Acoustat 6's, Marantz model 2's. McIntosh MI-75's.... ugh, Im gonna get sick if I keep thinking about it.
Oh bottom line, I imagine the Cornwall iii's and iv's are worth giving a listen to, but I would NOT commit to buy either until I had spent at least some time with them, especially at the 2021 price points.