Go for the 5 speed! I have a 64 model 4000 with 5 speed, and running things like a tiller would be all but impossible with the 4 speed (1st too fast). Also, 5 speed had 2 stage clutch giving you "live" pto!
Check out this site for MUCH more on these old girls...
http://members.aol.com/naa60512/naa.htm
My opinion,
Tommy D.