Unless they are truly four-channel recordings, they are more likely to be stereo "quarter-track" which simply means that the tapes could be recorded in stereo on both "sides" (fill the reel in one direction, turn it over and record again).
It works like this:
Left channel, side one
Right channel, side two
Right channel, side one
Left channel, side two
... so the left channel is always the top 1/4 of the tape and the right channel is always a little more than halfway down from there, regardless of which way the tape is running.
That's as distinguished from a half-track stereo tape, which can only be recorded in one direction or the other (if you fill it, then turn it over and record again, you've just "overwritten" your first recording).
Quarter-track stereo was a very common consumer tape format. Half-track was more professional (and had better signal-to-noise ratio among other advantages).
--best regards