Well I have not built this one yet and probably wont until spring when I redo the pepper plants. So I will try to explain it (not to hard). The 4 inch P.V.C. is not actually P.V.C. I think it's actually A.S.B. or something like that, but you can find it usually in the same isle as the P.V.C. It comes with pre drilled holes or in a solid tube (use the solid tube). It's 4 inches round (same as the bottles) and runs about $8 for a 10 foot piece. The end caps for this is about $2.
Just cut the tubing to the height you are wanting (cuts easy with a hack saw). The 2 liter bottles are about 10 in with the bottoms cut off. Then glue one end cap to one end. Drill a hole through the end cap in the center of it. Then glue in the P.V.C. connectors to the end caps. Now you have about the same size bottle, but you don't need to paint because the tube is black inside. Also it is structurally much more sound and durable for cleaning and reusing.
Attached are some pictures of the parts I am talking about, but make note of the bottom piece of the connector that threads into the top piece. In the picture you see threads to thread into another piece but should simply slip onto the P.V.C. tubing (I took the wrong picture). Depending on how much space there is you may need a spacer to make a sure connection between both top and bottom connectors, and the end cap. P.V.C. glue should be enough to make a water tight connection between all parts.
Also attached is a picture of a $2 heater filter that I striped the filter from and then cut to line the bottom of another system with to keep the
growing medium from getting down into the tubing. This can be used with these also.