If you go with 5 couples you cut your boat options substantially from 4. Starting and stopping in St Thomas also reduces options. I would suggest you ferry to west end and back for pickup and drop off. You will get more bang for your buck and free up boat time the first and last day. Your charter quote will be lower and you avoid the provisioning issue with a boat coming from the BVI to the USVI.
I think you can find a boat if you will be there prior to 20 Dec. After 20 Dec most boats will be reluctant to do a 4 day charter and probably have a 7 day minimum.
G