The 4K tranny... It's pretty close to the 2H from the MK1 NAM GTI.
I remember reading (and I still have the article from VW&Porsche) about the 020 transmission from 84-90 that had the self machining problem. I'm sure most were fixed or failed by now.
Here's a link with gear ratios:
Gear Up!
They switched to the 02A in the Corrado and B3 Passat (I think) and then for all with the MK3. Put a N/A MK2 diesel tranny in... It will sing like a B-52 in 5th gear.
I think every 020 will eventually get a bad 2nd gear syncro. My last diesel had that notorious grind in 2nd when it was cold if you didn't hold the cluth in for 2 seconds when cnanging gears. I solved that problem (band-aid fix that actually worked) by putting GM spec Corvette gear oil in...? Go figure?
If you haven't already swap the connecting rod for the shift linkage with the weighted MK3 rod. Night and day.
What else?