K.C.
She is well bonded to you. You respond to her cues like all moms should. And she will eventual grow out of nursing to sleep when she is weaned.
Don't over-think the situation. They all, in their own time, put themselves to sleep (think about this - if they didn't eventually sleep on their own then adults would need binkies and boobies to get to sleep from the day they are born till the day they die!). It is how humans are naturally wired. And for her, is it one-on-one time with mom and soothes her. Wearing her also frees you to spend time with your 4 year old.
Have you ever read about Attachment Parenting? It is basically what you are already doing now, but they have great sleeping advice. Attachment parenting advocates how humans have historically always acted with their young - babywearing, co-sleeping and breastfeeding.
Here is some info on sleeping from them: http://www.attachmentparenting.org/parentingtopics/babysl...