You can't use most of the strap mounting rear carriers because of the plastic spoiler/brakelight mounting on the rear ( most of them are pretty poor anyway).
I've recently bought a set of Cruz roof bars and 4 bike carriers from roofbox.com. They were a "kit" available for about 100 quid.
The only issue when fitted to the built in rails on my C4 (grand Picasso) was that the U bolt fastenings were too long, and came pretty close to scratching the roof, careful positioning, and in a couple of places reversing the bolt so the heads were "up" cured the problem.
We've just returned from Cornwall (700+ mile round trip) with 4 bikes on the roof with no problems whatsoever.