Need cleanout plug on septic line

I recently found (courtesy of a blockage in the outlet T on my first septic tank) that neither of my DWV cleanouts just outside of the house were capped. There are two cleanout pipes next to each other - one regular black 4" ABS, and the other very thin somewhat brittle 4" green sewer pipe. They both overflowed.

I went to the big box store and bought a black cleanout adapter and plug for the ABS. But I couldn't find something to cap off the green pipe. The very helpful worker in the plumbing section recommended forcing a section of black ABS onto the green pipe, and then putting on a regular cleanout fitting, but I was unable to get it jammed on.

By the way, the two lines seem to join together halfway to the septic, about 20' away from the house. At least that is what it seemed when I stuck the hose down both in turn. I can't figure out the purpose of the green pipe, since it apparently just goes from where it comes out the ground to where it joins the main line. Maybe it was used when the previous owner had a mobile home on the property.

Anyways, what is the proper way to cap off the green pipe? Why was there nothing at the store to do that with (in other words, is this not normal)?

You need an SDR to sch 40 bushing. Then the ABS cleanout will fit. It would be impossible to "jam" a piece of 4" ABS over it because they both have the same I.D. which is actually the reason the thin green pipe is "smaller" than the ABS. And any knowledgeable "helpful person" would have known that.