A slight problem

I think we may have a problem if baby #4 turns out to be a boy. Ella is absolutely convinced that it is a girl. So much so that she doesn't even call it a baby anymore, but calls it Sarah. The other day she walked up to me and said, "when Sarah comes out of your tummy. . ." and today we went shopping at The Children' Place and she went directly to the baby girl section and picked out a pair of pink striped pants and pink shirt and brought them to me and said, "our baby needs these!" She wanted no part of my explanation that our baby may be a boy. It took me 5 minutes to convince her to put them back! No I am starting to think that we may need to find out so we have time to prepare Ella if she is having a brother. What do you think? Should I find out and make the transition smooth, or just wait and let her deal with it in January?