hmm, M4 genitals are included with the M4 morphs. You can get several commercial products at renderotica , or a few freebies at maleposerotica. 

The part you mention is not part of any figure that I am aware of  but it is discussed in some forums. There was a thread here About it and a suggestion was made about doing one using a toroid.

There was a postwork kit that I believe included that part in the rotica store bt DrTaj.