I have 4 kids and love it now. I didn't feel happy though until about a year ago. I just found it drudgery and hard work (most of the time). My fourth child happened after dh's vasectomy so maybe the shock etc. of the pregnancy contributed to my feelings. Whereas your fourth will be planned so totally different feelings.
My children are now 12,9, 6 and 4 and I love them and love having four kids. I have 3 girls and youngest a boy. Also, I have a naturally lazy, unorganised disposition which is not a great trait when you have four other lives to organise.
Your 3 youngest will be best friends. I love watching my 3 youngest play. All my kids have someone to play with at any time really. I am sure your teenage daughter will be fine. My oldest doesn't need me as much as the younger two but she is not shy in coming forward when she does need a chat or to tell me about her day.
Do you have support (from extended family)? I didn't which is why I think I also found their babyhood, toddlerhood quite hard. Some days when they were much smaller it could take hours to get organised just to get out the door. I couldn't see the wood for the tree's then and could never imagine how happy and how much easier it was ever going to be.
Hope I haven't put too much of a negative slant on your post, just wanted to give you different opininions.
I also worry about having enough money and time to go round 4 children. I think you just find the money somehow and your time becomes like love, enough just comes to go around.
Oh and I wanted a fourth girls. I thought who could ask for anymore out of life than 3 sisters (no chance of being lonely). My friend is a fourth daughter and she loves it.