On Windows 7 64bit with BootCamp 5.1,
the same headset on the same combo port does not work with the driver as delivered by Apple with BootCamp 5.1 (more precise: 5.1.5640 ).
Apple phone support just told me it is not their problem,
because it works on OS X.
So I tried installing a better driver on my own.
According to the Windows Device Manager,
my sound card is a Cirrus Logic CS4206B (AB 07).
The closest match I could find on the Cirrus homepage was this one:
(Go to Resources -> Tools and Software).
The same page is also linked in this related thread, including a step-by-step installation guide, but it doesn't work for me. Did someone get a 4-pole headset to work including the microphone on an iMac with the Cirrus Logic CS4206B (AB 07) sound card?