After 2 years of use, the hitch, the connecting posts and the bearings have been changed http://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.
Step 1: Back assembly (rear unit)
1 Take the wheels A and the 2 connecting brackets B apart from the hand truck
2 Bolt these brackets on a piece of plywood C, which needs 4 holes for connection to the front unit