Interesting......
I have a K4 ecologic Neil
I have the K4 Ecologic (and hate it because it's fiddly and clumsy) but the Qwashers hose fits fine and is far better that the original hose. It's a pretty straight push-and-click into the gun end, but at the hose-reel end you have to get things lined up and then rotate the end of the hose until the flats on the connector line up with the socket so that it clicks all the way in. Only then will you be able to re-fit the spring clip. It's tricky the first time but no problem once you've got the hang of it. However, the Qwashers hose won't roll up on the hose-reel. I've put a B&Q garage hook up on the wall above where I store the Karcher so I can hang the hose up when not in use.
Note that you should make sure the pressure is released when you finish with the washer. Mine was in the garage during a really cold spell recently, the water in the washer froze, expanded and blew out at least one of the internal seals resulting in more water on the floor that through the washer. It's currently away being fixed under warranty.