The
Thule 830 Kayak Stacker will get the job done for you. It carries the kayaks on their sides, stacked up against each other so you'll have enough crossbar space to fit all 4 kayaks. You stack 2 kayaks on one side of the Thule 830 Stacker posts and 2 kayaks on the other. The Thule 830 Stacker does have a pair of rack straps that will allow you to tie down 2 boats on one side, howver you'll need to get another pair of rack straps to tie down the boats on the other side of the 830 Stacker.
A really cool product is the
Malone Easy Reach wheel step up. You just place this on one of the wheels and when you step up onto it you have super easy access to your roof top. It'll definitely make loading the kayaks and tying them down much much much easier!