This bike trailer can be used for transport of heavy or very long materials. It can also be used as handcart.
After 2 years of use, the hitch, the connecting posts and the bearings have been changed https://www.instructables.com/id/4-wheel-bike-trailer-version-2/
which makes the trailer MUCH better!
A 4 wheel cart, or trailer for behind the bike is more complicated to build than a simple 2 wheel model, which can be rigid. The front wheel assembly needs the ability to turn, the hitch has to be able to tilt up and down, and suspension should be used to allow the front wheels to follow the unevenness of the road independently from the ones in the back.
I built the trailer in less than a week, including painting.
The costs were about E 55 ($ 80)
Needed: 2 hand trucks with inflatable wheels
Some plywood, preferably water resistant
About 3 m of wood , like 40x70 mm or 10 ft of 2x4�
2 (broken) bike seats, for the springs
Nuts and bolts, and lots of washers
Steel central heating pipe 22mm, 2 elbow compression fittings and one tee
4 teflon furniture gliders
Tools: Drill, pipe bender, wrenches, hacksaw, saw.