I must have been crazy! When I found out we were having an unexpected DC4, I keep saying 'well the baby will have to slot in with us' and we will continue on as normal, another one won't make much difference'. 
Well now our Lord and Master is 19 months old and I can't get anything bloody well done! The house has fallen to rack and ruin. The ironing pile is up to the ceiling and I absolutely knackered as he still does not sleep through consistently (other DCs not affected as he sleeps in our room). My day centres around taking him out as he will wait at the front door shouting 'shoos' if we are at home for more than an hour!
As much as I adore him, I feel I am just too old for this (40!) and am finding it harder work than when I had toddler twins! Anyone else find it harder with a big age gap (8 yrs +) and being older?