All four channels are carried over in the same 5-pin DIN cable. However, the cables that actually have wiring for all four channels should be noticeably thicker. While quadraphony went out of fashion, I believe the option became more theoretic towards the end of the production run, and only 2-channel wiring may have been installed.
I think opening the top and taking a look is the only sure way to know the decoder has been installed - but the thinner cable and/or a missing "CD4" switch under the right edge should confirm it is not there!