[Analogue man]

Normally 8 track players move the replay head up and down the tape to pick up the various tracks, there are three possibilities here as I see it, one is poor head alignment in relation to the top track on the tape or that there is a disconnected wire on the head which touches its terminal when track two three and four are selected but stops touching when the head goes to track one or when the head reaches track one perhaps something shorts out the connections for the left channel.