How would you use a half-inch four track machine?

[zorf]

Or, record the harmonicats each with their own channel.

Or, the mills bros.

Or, any four person do wop or vocal group,

Or, four tracks of kazoo.

 

[Muckelroy]

Assuming it's got a separate record and repo head, take advantage of that gap and use it to make some delay/echo effects. Looping back the output of a pre-recorded track into the input of the adjacent track and feeding a slight amount back into the first track in a loop - makes for the basis of most any tape echo system, and makes some REALLY cool effects once you start toying around with the feedback levels.