We flew over to Geneva and hired a Merc Vito 9 seater, took the rear seats out and unfortunately had to turn them round against the middle row (hire place didn’t have room to store them). Managed to get 6 bike boxes/bags with road bikes in, 6 mid sized suitcases/holdalls and 6 people in. That was a pretty tight fit and pretty uncomfortable. Would have been a lot better with rear seats out though.
Whatever van you hire you’ll have three seats across the front so you’ll have two spare seats to stash luggage on. Getting 4 bikes into the back of any of the crew cabs will be easy enough, some may need front wheels off, some won’t.
I’ve got a crew transit. Not sure I’d want to do such a long trip in it but it’d manage it easy enough. There would be enough room in the transit/trafic/others that size for 6 of you and gear. Might be a bit uncomfortable for the one in the front middle.
For 4 of you any of the crewies will do the job with ease. Vito/Transporter will probably be the most comfortable, Trafic/Primastar/Vivaro won’t be far behind and the Transit is a bit more aimed as a work van rather than a leisure van. Bigger van means you can bring a spare bike or two though!
People carrier with a tow bar rack will probably be a lot cheaper though. Roof mounting them will probably see horrendously low mpg and higher ferry crossing prices too.
One other point to consider, the tolls across France seriously mount up. If I remember right a van will attract a higher charge so remember to add that into your sums. Think we were well over £50 each (two of us) to get a car to Morzine three years ago. As the Vito was a people carrier on the last trip I think it was classed as a car – may be wrong on that though as I wasn’t driving or in the front.