Quad fin set ups are an exciting mix of both the thruster configuration and the more traditional twin fin.
The side fins are set in the normal position but instead of having a center fin in the back position, quads have two more side fins which are called 'trailers' because they trail the front fins.
Quad sets are usually very fast and offer great release through turns, but do feel 'looser' than thruster set ups. Quads are a great choice in fast, barreling waves where the main aim is to get from point A to point B quickly!
Quads are either sold as a set of four or as a trailer set which will just include the 2 trailing fins. Also, many modern short boards will come with five fin boxes, giving the rider the option between using it as a thruster or a quad, in which case it is best to buy a tri/quad set.