The same way you explain it to children of divorce, etc: things happened in x-way and people had to do x-thing at x-time. I'm guessing from the sounds of things that your son's firstmom wasn't ready to parent when she placed him with you. While it doesn't sound like things have changed enormously, or she wouldn't have mentioned placing with you, she has grown and matured in four years. Children, in general, change a parents' (biological or adoptive) priorities. It is possible in the wake of the adoption, four years and subsequent pregnancy, that her priorities have changed enough to allow her to be a great parent. To a child, all you have to do is say that she did what she thought was right for him at the time.
Many firstparents have children after they place. My Son will be raised knowing and interacting with his sister though she lives in a different state with different parents. Munchkin will know her brother.
And hopefully love him in that siblings-are-dumb kind of way.
