Nah - replacing the tower weight only a temporary fix as problem came back after a while, so not the root cause. But have now found a quick and cheap fix for the sticky fourth to third gear change problem for 02T gearboxes. Might be worth a try for the other gearboxes too.
Picked up a cheap deadstock replacement shift tower unit and was looking inside to see how it works. The two tubes on the base of the tower hold spring loaded ball bearings which keep the shift column in place and guide it through the gate movements. Aside from the sleeve bearing these are another moving part that needs lubrication so figured could have something to do with the intermittent sticky gear change.
The breather vent on the top of the tower sits above these bearings so just pull off the black plastic vent cover and use a small funnel to slowly add some gear oil. Work the gears through the gate, add a bit more oil and repeat a few times.
Did this about 4 weeks ago and gear change immediately back to normal. Will see how long it lasts and if same fix works again. If not time to fit the new tower.