WMP, I understand where you are coming from. It's not that you couldn't love a boy it's just that you feel SAFE with another girl, I'm sorry but who could really balme you for that you have 3 of them.
As for the sister bond, Umm that baby if a boy will be spoilt rotten by it's big sisters that is a fact.
My ds and dd are just adorable together, I am pg with no.3 now and will not be upsett either way but if I'm honest from a practical point I would like another girl. Only being practical, my ds is adorable and was a cuddly, scrummy bundle of fun, far more than dd who is independant, miserable, moody and sometimes she is already a B*h, she's only 18mths old! However I love the girly bit and we have fun doing Pretty things already.
I also agree with soupdragon and the others, it doesn't matter about anybody elses stuggles, MN is here for us to post OUR worries whatever thay may be, some may be big things and terible terrible experiences but not every one has these and poeple shouldn't feel bad to post smaller wories in fear of getting a "well, you should be greatful you are pg or that the baby is healthy" comment.
There have been some great posts on here, I hope you can take them and feel a little more positive, if the scan today shows a boy then it may take time to get used to the idea but you WILL without doubt forget all about your worries when that beautiful boy is placed in your arms. He will be different and bring some change but he will make you smile, cry, laugh and worry the same as your 3 girls do now.