Hi Paul,
While I know nothing about the Xtrail, I do know a bit about Land Rovers.
The Series 1, 2 and 3 Land Rovers (i.e. those made up to 1983) were selectable 4wd, while those made after that date (Defender) were permanent 4wd (like the Range Rover).
The permanent 4wd system includes a centre differential, which allows front and rear axles to turn at different speeds (eg when cornering), but which can be locked when needed (the diff will send the drive down the path of least resistance so, if one axle loses traction the centre diff-lock will ensure that drive still goes to the other axle).
With selectable 4wd (at least with Land Rovers) there is no centre diff, so drive will be distributed equally between the two axles when in 4wd mode. This is far from ideal on the road, as the axles need to rotate at different speeds according to the conditions and driving in this way will cause the transmission to "wind up".
When using 4wd off-road the stresses are dissipated more effectively by mud and bumps etc!
So, in short, you risk damage to your transmission if you keep in 4wd on the road - keep it in 2wd.
I expect someone more knowledgeable than me will now tell you that the Xtrail is completely different from the Land Rover!!