Disclosure: I have no TYM433 experience. And I don't know your experience with manual transmissions, so don't be insulted.
:2cents: As per tractordata.com: "Four synchronized gears (1-4) in four ranges with synchronized shuttle shift. Tractor must be stopped to shift range."
I assume you're talking about the gears 1-4, not the ranges? These should be synchronized, so stopping shouldn't be necessary.
That being said, try some "old school" shifting like one has to do when a transmission doesn't have synchronous gears:
Typically when one upshifts it's easier because you typically back off the throttle (yes?), shift up, and now the lower engine rpm in higher gear nearly matches your existing ground speed = smooth shift.
On the downshift you have to make it do the opposite: Lower engine rpm in higher gear prior to shifting, clutch in, shift lever to neutral, rev the engine UP to match (or slightly above) the engine rpm that will be required to match your ground speed in the lower gear, then shift into lower gear, release clutch.
Better yet, double clutch it: Same as above, but after clutching and bringing shift lever into a neutral spot, release clutch when in neutral, rev. engine up to match (or slightly above) the future lower gear's corresponding engine rpm (at your present ground speed), then clutch in again, place in lower gear, release clutch. All of this can be done so total shift time is about <1 second.
Edit: I should clarify that when I say "rev engine up", I mean bump it up and release, don't hold rpms there. You're looking to find that one special engine rpm that corresponds to ground speed in future gear where transmission will go in smoothly. Because you don't know exactly what that rpm is, by bumping and letting off, you can "find it" as engine spins down.
It's easier to do than explain!
...and like the saying goes:
IF YOU CAN'T FIND IT, GRIND IT!" - No, not really, don't do that!