Hi! Welcome to the forum.
You seem to be happy and excited about your life, cool! There was a time in my life when I was dreaming about a similar situation than yours. It never happened to me, but I can imagine that when it works it can be great.
As for your questions, there is this huge "Coming Out" thread here, http://www.polyamory.com/forum/showthread.php?t=164
Most likely useful for you thinking of how to tell other people about your situation. I am in that too - in the process of coming out. Started with telling those friends that I can count on will love me no matter what, and it was a good decision. Now three people outside my polyship know about us, and they have been very supportive and happy for my happiness.
Then to my opinion, telling a four-year-old is much simpler. You just tell him that the baby has the same father as he does so that makes the baby his sibling. Children are highly adjustable to new situations and new thoughts and concepts - they are still learning the world. And, as he must be by now used to having "auntie" as a family member... should not be too big a deal.