"Cam belt/tensioner life of 4 years. Why?"
Because they are/were a bit crap underengineered.
Can't argue with that. I think in this day and age the cam turning mechanism should last the life of the car. Very poor engineering I am afraid.
We had company Vectras a few years back. I asked a guy to go to a meeting in Cardiff in my place. His car was due 80,000 mile service that day. The cambelt gave up at 80,040 miles. Cost just over £1500 to fix.
Cant see how that possible on anything other than a cog driven setup (adn that would chatter with time).
Chains come close but still stretch and wear teh guides plus they need to be in oil (and the regularity of the oil changes has a big impact on thier life).
A cambelt setup is generaly quieter but, no standalone bearing will last the life of an engine and I suspect that actualy, the Vx setups would do 60K miles but, is it worth the risk (as the date will be based on an acceptable failure rate). Note, the tensioners etc do use good bearings to, its not like they are cheap chinese things.
I note that even the likes of VW and Alfa etc (in fact, most of them) have shortened thier cambelt change intervals.
I also suspect that other than life, the bioggest cause of belt failure is poor fitting (of which we see all to many!).
Its an engineering compromise.
I know I for one personaly prefer a belt setup to a chain as I know it will be changed and when it was changed