How long have you been married to have a 4 month old stepson?
I adopted a child at birth and allowed the bio-father's parents to have the pleasure of her company every other week end, immediately after birth. It was so hard. I grieved all week end. I love my daughter (now 4) enough to share her with the whole family. Of course now she doesn't want to be away from Mommy, and I love that even more than sharing her.
However, it takes 9months to have a baby, it's 4 months old. Wow, Normally relationship hopping doesn't last. Good Luck.
Sounds like you need it.