I've been enjoying your posts! Glad to hear things are going so well.
If you're looking for some support, my vote is Yes: Go ahead and get a 4th sexual partner!! Yay!
But, more seriously, I do understand your slight feeling of guilt & responsibility toward your husband and C. You should probably talk to C more deeply about your potential new partner and how he would feel.
I also understand your anxiety when C goes on dates. My only partner (right now) is looking for someone to be more "primary" than me (which I totally support), and there is a possibility he will want to be monogamous with such a partner someday. The uncertainty is sometimes hard...if there will still be room in his life for me...but, on another level it also feels totally right to be able to go forward with no expectations. I'm happy.
Single, straight, female, solo, non-monogamous.