The optic that is installed in IM4 is SM2-01, and from the document T1200140-v1 I've identified it's transmission to be roughly 5900ppm, p-pol.  For a 160uW incident beam, the transmission would be approx. 1uW.  The transmitted beam is split evenly between IM4 Trans QPD and the ISS array, so at 160uW incident on IM4, each of the two transmitted beam paths has about 0.5uW of laser power.