I felt like I "knew" we were supposed to call an attorney about adopting. I wasn't supposed to be able to have anymore by birth (had my 4th son anyway, but that's another story). I told my dh and he told me just to call and inquire. Long story, but dh knew what her name was to be, and said it out loud 3 days before she was born. She was in our arms 9 days after I called the attorney to "inquire." lol
All that to say, God has a way of making it plain, but sometimes we have to take the first step by faith. If you know there is a 4th child for you, now is as good as later. I agree with Remudamom - financially another baby probably never makes sense! If your dh is on board, I say go for it.