We are having #4 in August and I was the youngest of 4 myself. I like that my kids always have someone around to play with, although it can turn ugly pretty quickly at times.
There's no down time, washing never ends even just with 3 and being pregnant with 3 kids is just so damn hard some days (to the extent you can plan these things try and have the latter part of the pregnancy in winter - I'm only 13 weeks but summer has nearly killed me this time - I can't imagine being 8 and a half months!). I have had 2 summer babies and it was rough.
Holidays are hard we went overseas with the 3 in January and I cannot fathom doing it with 4, but we will as o/s travel is important to us. It will just happen less often and be expensive.
Educating can be expensive if you want to go private. I am not looking forward to trying to manage DD1's homework with a colicky newborn or the sleep deprivation and still having to do school runs, canteen duty etc.
But my kids are awesome. I can't wait to meet the next one.