First check the condition of the components. Do the Ujs on the linkage feel loose? There are springs under the rubber covers, these can break. Is the rubber joint on the shaft in good condition? Are you sure you are getting third gear? Driven on the road? There are two side stays that can be adjusted, one on the transmission across to the suspension turret and one at the left rear cylinder head to the inner wing. When you manually rotated the shaft to get 4th gear did you have to exert any length-ways pressure to get it? Adjust the transmission stay first towards the transmission to see if helps. If not back off the other way. Adjustments are usually of a small nature before the problem is resolved. If this does not resolve the problem the the column can be adjusted but this can be difficult if your vehicle has the steering lock.
Ive been into 3rd when driving the car, when you change to 4th it feels as if it the lever isnt quite going down far enough to engage. The lever is at the bottom of its travel.
How much play should there be in the universal joints? There is a bit when you wiggle them. The rubber joint looks in good condition and is intact. The spring on the first universal joint is floating about and not attached to anything, is this normal ( is the spring there to keep the rubber cover away from the joint or does it provided assistance to the gearchange? )
Ive got access to a spare steering column and gearlinkage, but will try the adjustment first.