I've had both and I'm in the Fuse now.
I prefer the positive foam footblock over the happy feets and the adjustable thigh hooks as opposed to the flatter and non adjustable thigh non-hooks of the 4Fun. I also prefer a backband that supports my lower back. The backband of the Fuse is firm and stays put.
Why I left the 4Fun?
One occasion when I'd roll up my thigh would slip out of the brace and nearly pop the skirt. I'd pull myself back into the boat get my thigh back in the brace and roll up. I guess this is because the thigh brace is pretty flat and not really hooked. I found it hard to get and stay tight in the boat. Probably would have been better without the happy seat and feet. River running I found the Fuse to have a less catchy stern. I also had a couple of times where the rope in the cleat of the 4Fun back band would "pop" and I'd lose my backband till I got to the bottom of the rapid and reset it. A bit unnerving and my fault for not setting the rope firmly in the cleat.
I have an AllStar for play but for all around I prefer the Fuse.
My 2 cents.
"Let us cross the river to the other side and rest beneath the shade of the trees." ~ Last words of Thomas Jonathan ''Stonewall' Jackson