How do you DOWN shift from 3 to 4? What kind of lubricant are you using? If someone has convinced you to use synthetic lube, the synchronizers probably can't get a grip. Be sure the clutch is adjusted correctly and there's no hesitation or grind when you try to engage a gear from a full stop.

Assuming you're running a SM-420 truck transmission (not a Muncie or a Saginaw car trans) I hope you're aware there's no way to downshift to 1st. while moving- - - -that one's not synchronized.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!