There was a company around forever that made 4 foot sickles for garden tractors... Haban Sickle Bar Mower Attachment... I don't know if they are still around but I'm sure you can still find the mowers, I bought a new one a few years ago, I was going to adapt it to my Cub Cadet 1450... But never got around to it. I have since put a 7 foot on my Kubota L4200.
I think Ken Sweet is on the right track with a 5 foot for your needs... 4 foot is just a bit short... I find myself at times wishing I had located a 9 foot... KennyV