While it could be a transmission issue, first thing I would check would be the shifting components on the console. When my dad bought our 4020, we thought it had a bad transmission since we could only get it into a few gears (and by a few I mean 2 forward and 1 reverse). We I got to checking it out, I noticed that there was so much sand, dirt, grease and leaves clogging up the collars (see items 10 and 16 below) that it wasn't letting the lever move. After 15 minutes of cleaning and putting it back together, our 4020 has been shifting fine for years.
It does overshoot 8th gear, but I think that's due to the groove worn in the shift lever and possibly the linkage as Tim S mentioned below.