It's not possible to use a 4pets dog crate on the back seats of your car, they are just designed to be used in the boot.
In order to be secure the 4pets dog crates they need to be pushed up against the back of the rear seats and secured to the anchor points on the boot floor using the supplied straps. You wouldn't be able to do this if they were on the back seats, so we are not able to recommend using the 4pets cages either on the back seats, or in the boot when you have the rear seats folded/removed.