“Watching my three cats, I’ve noticed that they can, with little effort, jump as high as 4 or 5 feet, from standing still (no running jumps).”
I doubt that. There is an upper limit on how far any animal can jump, and it’s about 2 feet IIRC. Any ability to clear higher objects involves scrambling or, as in the case of human high jumpers, shifting the centre of gravity. Your cat might be able to clear 4 feet from a standing start by lifting 2 feet, stretching up to 4 and then pivoting around the obstacle, but they can’t really ‘jump’ 4 feet.
Since their ability relies entirely on their ability to twist in the air you wouldn’t gain any ability from having catlike muscles in a human body.