4 people will be crowded on almost any beach catamaran. You will have to add a forward trampoline to occomodate the extra people.

Otherwise- the Hobie 18, as mentioned above, would be an excellent choice.

The G-Cat 5.0M or 5.7M would allow you to carry more people, and I've not had trouble keeping up with Hobie 18s on my 5.7M. The only difficulty with this boat is finding one in your area. There aren't so many out there, and the owners aren't easily persuaded to sell!