Ok well it could still be the bias plate that needs adjusting slightly, or it might be something deeper in the 'box.
To adjust the bias plate, take out the centre console/insulation/rubber membrane until you can see the top of the gearbox.
Remove the alloy cover that the gear lever goes into and then you'll see the bias plate and it's two springs. It is held in place by 4x 10mm bolts, slacken these a bit so you can move the bias plate until you get a smooth selection of 4th. By the sound of it you will need to move it to the right as far as it will go. Then re-tighten the securing bolts and road test. Assuming it has fixed the problem, put everything back together.