Replaced clutch disc release near top after adjustment

Which linkage or rod would be slotted.

If you have the standard Z bar you would make the bottom end a slotted design instead of just a hole.
That way you could adjust the geometry any time you wanted.
Basically you could change how much the TO bearing fork moved compared to how much you pushed the pedal down.

I could do it, but I'm not so sure just anyone could.

On the problem with fourth you probably just need to get your shifter rods adjusted, as fourth should be the easiest to get into.
Does it fight the actual engagement, or does the shifter not like moving to that position?