A trailer is what you need for 6 bikes chap even 4 is pushing it.
I do beleive a Fiamma one can be extended to take 4 [i am possibly wrong though] the big problem is that putting that much weight on the tailgate would not be good news putting far to much strain on it .....
A towbar mounted one might be the answer though as the forces are being put on a far sturdier location...
T4/T5 Club UK and its domains are currently up for sale due to severe illness of its owner, if you beleive you have what it takes to run it get in touch TODAY (note the community is NOT at risk and will continue in any situation)