When looking through a datasheet for a connector how do I find out which pin on the part does what?
The data sheet extract you have shown gives pin numbers but no diagram of the pin functions. Examination of the remainder of the datasheet doesn't reveal any further information.
Your best option is to insert a 3.5 mm stereo jack into the socket and determine the pinout by continuity test.
 
Figure 1 and 2. From here.
Since there are four terminals there may be a mic contact (4-pole connector in Figure 1) or a switch contact that closes to indicate that the plug has been removed (a cut-down version of Figure 2). You can determine this by continuity test also.
I think it might be worthwhile to point out to them that their datasheet is lacking some basic information. I was buying their jack plugs and sockets over 40 years ago so they're on the go a long time and have some reputation to maintain.