judging from past posts, this is not an uncommon experience. I would suggest that as all gear positions can be found, it is not "mis-timing" of the gear selector that is leading to the loss of 4th gear under normal operation, but the sum total of the end float of the three shafts; main, lay and gear selector, allowing a misalignment; the float is permitting the shafts to move under load when shifting gears rather than the shift selector properly positioning the gears themselves...
It also seems that the paper gaskets between the three gearcase sections are critical, acting as shims for achieving proper spacing of the mechanism. Binding can occur without the gaskets, such as when attempting to use a sealer in lieu of...