A Tepui Ruggedized XL would be a perfect tent for 4 people. I have one on my trailer. For your double cab it will be a little longer than the bed as it has a 4x6' closed footprint but if you don't mind it hanging over the cab just a little and maybe a little off the back it would be a great tent. I'm a dealer and can make you a great deal on the tent if you're interested.
The XL is the one on my trailer in this photo
As for the mounting. There are numerous people that are using roof rail tracks from various vehicles or using some from roofrack companies like Thule. I know that Thule will have rails that can be mounted to your topper and will allow for the use of their load bars. Of course you could also go with something custom. 4 people is a good amount of weight on a fiberglass topper. Most toppers are not designed with that much weight in mind and it might be worth your time to have some extra bracing built to go on the inside of the topper. There are a few different trucks on here that have done that so a search might yield some ideas. Basically if I were doing it I would build essentially an internal cage in the bed with plates that sandwich the roof of the topper and the rack on the outside would bolt through to the plates on the inside. It could be pretty simple and not take up a lot of valuable space, but the little bit of extra support would be a good thing for piece of mind in handling the weight of the tent+4 people.
Good luck, and post up pics of whatever you decide to do.
