I am always pre-thinking any possibility, so I built in an extra seat on my M-15 before I even thought about four in the cockpit. Using wood and marine aluminum chair posts, I installed a hard plastic stadium folding chair in the companionway. With the hatch slid forward you can seat one (heaviest) person in the companionway while seating the two other people close to the cabin house on port and starboard. This leaves enough room for you to control the tiller and mainsheet, while the passengers work the port and starboard jib sheets. Everyone has a position and the boat remains solidly balanced. You can also run the main sheet to the back of the cockpit using a couple of pulleys and another jam cleat. Unlike some others who post, I usually sail alone and I have only tried out my system once, with four, in a fairly good blow. The boat handled very well with the extra weight and had good stability in the wind and waves. It even works quite well for just two people since the captain can have the cockpit to his or her self and not have to worry about someone else switching sides on each tack. I'm posting this so someone else might benefit from it and feel a little more comfortable with carrying four on board. It was not that expensive or difficult to make and it is removable when you don't wish to have it in the way. My wife refuses to sit in it , she calls it my red neck chair! Monty 15..."Still The One"... Lou Aliotta