Congratulations! Your glass isn't half-empty or half-full, it sounds like you are blessed with a full glass!
In other words, it's all in how you view your situation.
How does one manage this number? By taking what looks like a very unmanageable situation and breaking it down into smaller pieces. Do what you can with what you can. Take control of that which you can have control over. Don't worry about the rest. Worrying about tomorrow ruins today. Can you start teaching your baby to start putting herself back to sleep when she wakes in the night? Is it hard? Yes, but having her fall back asleep on her own is priceless! (She doesn't need to nurse multiple times at night.)
Start working on your marriage? Read "The Proper Care and Feeding of Husbands" when you are in the bathroom, sitting in the car in the school pick-up line, or whenever you have 10 min. here or there. When you take care of your husband, he will become more helpful now that you will really need him. (Just don't tell him you are doing it.)
Start working on your finances. Make the decision to not eat out, buy prepared foods, or whatever you are spending the extra money on. (We all have things we indulge ourselves with.) It will be so worth it when this new precious bundle of joy arrives. Make double meals so you can freeze one for those hectic days.
Ask for help. After the baby is born, when family and friends ask what they can do to help, don't say, "I don't know." Start a list now of things to say if they ask. Remind yourself that THEY are offering. Trust that they love you and want to help. Suggestions: Would you mind making an extra meal when you are cooking sometime this week? Would you mind picking up some milk for me when you go to the store next time? Would you mind having ___ over for an hour or so sometime to play with your son/daughter? They always have the opportunity to say no.
Do you judge people for having too many kids? Then there probably aren't too many out there that do. If you come across someone who says something like, "Wow, are these all yours?", just assume that they are just making an observation about the fact that they couldn't have four kids. Just answer with, "Yes, and it is more fun than I ever imagined!" They will be shocked and you will have had the fun of shocking them!
Look at pictures of what the baby looks like at each stage of your pregnancy. Then you might not hate being pregnant so much. God chose you to grow this new life. What a privilege! Many out there will never get that opportunity. Be sure to talk to this new one with joyful anticipation, too.
Be excited! The more you think of the joys that come with a new baby, the more you will be anticipating instead of worrying.