From Sweetgrass website:
Whether your rod has 4, 5 or 6 sides is a decision about which much has been written and about which passions are strong. We would encourage you to conduct your own research into the pros and cons to formulate your own opinion.
That said, here is a general overview.
Hex rods are the most traditional rod and are a good place to begin a collection. The action is smooth and the bamboo fibers do much of the work for you.
Pents offer a bit more reserve power than a hex, and the wraps on our pents are in a garnet color that many find both striking and distinguished.
Quads represent the "power" rod configuration in our line-up. Some say they help one cast a fly in a straighter line, and theoretically, all other conditions being equal, this is likely true. But, the number of variables one must keep equal is immense--wind, fly size, rod length, temperature, etc....so, don't expect that your casting will advance to "expert" level just because you are fishing a quad.