If you are only going to a one water park it is cheaper just to get a ticket ($33 for adults, vs 45 to get the plus.)
Also you can just get them there if you want to go. My friend got pluses on her tickets last time and felt the NEEDed to go to BB becuase it was already paid for (and the weather wasn't that great.) They wouldn't of if they hadn't already bought their tickets.
At 4 the best water park would be BB. It is fun, there really is a lot to do for the kids. TL is good, but for kids a little bit older and it is very splashy. If she doesn't like lots of water in her face, BB is better. (Our DD has been to both and likes BB best.)
I am guessing you can go with out it and have a perfectly fun time too. (and more money for something else.)
I would wait and see how the trip is going. If your 4 year old REALLy likes water play it might be worth it. Also if it is very warm and sunny.
Last year we went with 2 just under 3 years olds and it really was the best day of the whole trip. They even had more fun then at MK. This year we skipped it, it was fine.
BTW, mom's with new babies go all the time and are just fine. (depending on how NEW you are talking about.)
However a trip to WDW with Grandma and Uncle can be a very speical thing for a child who just became a big sister!
