Check the bushings at both ends of the shift linkage rod for wear. You can also disconnect the rod at the transmission end and manually move the shift lever to check for freeplay if the bushings are ok. I've replaced a lot of bushings but I'm wondering why the problem only occurs between 3-4. If the bushings are worn( or even missing ) they can be replaced easily. It can be a bit of a bear to install the one at the shifter due to the confined work space, but the lever on the side of the transmission can be easily removed and the bushing replaced at the workbench. Let me know how it goes.