Adding my 4th was harder, for sure, but it wasn't any harder going from 1 to 3 (twins). Mine are all very close in age (3,1,1 when my youngest was born). Things that were harder- a double stroller was not even close to enough. I used to use the double stroller for 1 twin plus baggage, then sling the other and walk with my oldest- NOW, I have 4 in cloth diapers, need at least 2 bags for a normal outing, always have one in the sling and no storage for my bags. If someone needs a potty break or has an accident, it is just mad chaos. My van is SO stuffed with carseats and crap, I have 3 cribs in my house (though there is a lot of co-sleeping still going on). Now that there are 4, I can't get out of the house with less than 40 minutes of prep. I probably sound a lot like debbie downer here, and it's NOT as hard as I am making it sound, but it is for sure a lot of work sometimes. I will say though, now that my kids are a little older (4,2,2,almost 1), they DO play together great (with the expected bickering and fighting)! I also believe that there is a 'herd effect' that happens wiht 4+ kids. We have playdates and I babysit my 2 y/o nephew (and for a while we have a roomate with a 2 year old also), and it didn't seem like it was a big deal- things just flowed together, if that makes any sense.