I would think the length of the trailer's tongue will be your only issue. It's a bit short on mine, so I have to be careful not to let it jack-knife, or I could get a bow through a back window. But if yours is long enough to get it comfortably far behind the car, the 6 foot span should be plenty to support the boat.
Yup. I thought long and hard about the same thing since I already own a similar 5x8 util trailer, but my problem is that the tounge is too short. With 2.5' of bow extending beyond the front deck, it would interfere when when turning anything but the slightest amount as Indy pointed out (it needs to be about 3' longer than the tips of the hulls extending past the deck). IMO, w/ the shorter 4x6 bed, you may need to have the tounge extended to allow that critical turning clearance.
But I finally broke down and bought a new Karavan bunk trailer ($625) from my local powerboat dealer, and purchased the catamaran conversion kit directly from karavantrailers.com online parts sales. Comes with mast stand, rollers, brackets, hardware, crossbars, lighting pieces, everything you need to convert just about any common bunk trailer to catamaran use.
Total cost fro trailer and kit was just under $1100, about same as Carnai Wave trailer(w/ optional maststand), but with the added bonus of larger 12" wheels, a winch, rollers, and the larger frame to accomdate a Getaway if I wanted to upgrade in the future. My crossbars are spaced about 6' apart, so the span your are looking at with the 4x6 trailer is fine.
Check out the gallery in a thread posted by Sailfast24, http://www.hobiecat.com/community/viewtopic.php?t=10776
my trailer is basically identical, same model and kit, with the exception of all the cool stuff he added to trick his out. Oh, and with mine, I mounted the crossbars UNDER the frame, lowering the whole cat 5". Plenty of clearance for fenders in case you were wondering.