It has been years since I worked on the brakes on a JD 4420. There where very few around here. I don't remember them being that hard to work on.
I would bet that you need to just pull the left transmission output shaft out of the transmission to be able to drop the brake assembly down rather than taking the brakes off the end of the shaft. That is how the left brakes are on the JD 6600 and 6620s.