Go to a motorcaravan specialist. Fiamma make very good racks that attach to T4 and other vans. They need specialist fitting but are very good. Google for the Fiamma website. I've used these racks on 4 different campervans and had 4 bikes on them.
We have a Fiamma on our van. They are good value, but the bucles and bits do degrade with time (years) but luckily they are all replaceable at reasonable cost.
The security is only as good as your lock. You can get cable locks that jam in the door which are more secure than just securing the bikes to theselves and the flimsy aluminium rack, but if we are leaving the van unattended in a city we have a big chain and thick extension cable that goes round a solid bit of the van underneath.