Plans are not necessary. It is drop dead simple, if perhaps financially daunting, to build a quad, and Utah Trikes sells everything needed. Step One: buy the tadpole of your choice. Used, obviously. Step Two: purchase one of
these. The resourceful can cannibalize a Sun Delta obtained very cheaply (it is the exact same item). Step Three: finish out the build with chain extension, rear wheels, rear brakes, etc. Step Four: Enjoy your quad. This is the only sane way to do this without the requisite background in metalworking that people like the Atomic Zombie principal has in his skillset.