If you open it up, it would be worth it to take a look at the clutch keys and springs of the 3rd-4th sliding gear.
My 4speed had trouble going into 3rd without double clutching or just waiting really long for shifts... Upon inspection, the synchro is not worn much and I think the most improvement will come from replacing the clutch keys and springs. They are what provides thrust force when you press the synchro, and if too weak will not push enough to match speeds.
I have not re-installed the transmission yet, but measuring the detent thrust force on the sliding gear, I've seen an increase from ~30 up to 50lb after replacing the keys and springs.
