I figured it out.
For posterity:

computer-side: speaker --> radio audio out
computer-side: mic --> computer audio in

-------- Forwarded Message --------
Subject: 4S-Link question
Date: Thu, 02 Apr 2015 17:30:41 -0500
From: Chris Howard w0ep <w0ep@...>
To: 4sqrp@...

I bought a 4S-Link already assembled, from another gentleman.

I'm looking at the schematic and the instructions online.
The schematic shows two connections to the computer:
headphones and mic. And three connections to the
radio: key, mic, speaker.

The unit itself has one side labeled computer with
mic and speaker ports; one side labeled transceiver
with three ports key, mic, phones.

I'm confused about the transciever-side:phones
and the computer-side:speaker.

I guess I could open the thing up and trace out
the circuit, but I'm hoping someone can just
tell me which hole is for my computer audio output
and which hole for my radio audio output.

Thanks all,

Chris