Custom Recumbent Catrike Pedal Quad
 

I came across a video of a 4 wheel Custom Catrike/Surly Pedal Quad Recumbant, after further looking around it looks like this bike was built by the Utah Trikes company.

Seeing how I don't have that kind of cash to drop on a purchase I was hoping I could find plans to build such a bike.

Anyone know of any resources that might be helpful in a design layout?

I have a tadpole and could consider putting a tricycle rear end on it. Looking at tadpoles and tricycles, and envisaging that, it'd be quite easy to design it and build it from scratch, alternatively.

But there's a problem. Three wheels are always touching the ground with a trike. Unfortunately, that's also the case with a rigid quad, unless either the ground is perfectly flat or it has some sort of axle articulation - namely, independent suspension on at least one axle.

Look around https://www.atomiczombie.com/
Have fun building, but I think you'll be surprised at how good a deal production bikes/trikes/quads really are.

A few years back, I was looking at the Atomiczombie stuff and found it was decent plans.

My other crazy passion was flying and building a high wing airplane. Hence oxy/acetylene welding, 4130 aircraft tubing. You can braze 4130 tubings and have a robust joint.
Bicycle tubing is double butted (outside the same diameter, inside diameter shrinks down and ends) for a sturdy frame but even using plain old steel tubing is excellent for a prototype "does it work."
designs.

The cheapest way to parts is as Atomizombie mentions, get old scraped bicycles and cut and weld your own.

For rapid prototyping, carboard and hot glue guns are fast. Does the linkage work, does it bind, range of motion? Once you have a working model, make it from metal and go to town building.

Thankfully my kids were at my elbow learning skills. I had read about Caroll Shelby oxy/acetylene welding aluminum. Couldn't get the hang of it. A guy, Kent White - aka Tinman - taught four-day classes, so grabbed my son and we went. Later, another son went to his classes and works as a fabricator (He also attended the two-week bicycle mechanics training course in Ashland) These days they are fabricating all sort of things, including mountains bikes.

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.