I found going from 0 to 1 the hardest. #2 slotted in quite easily. #3 and #4 were difficult for a number of reasons, mainly because my son was ill and that put a lot of demands on the entire family. Also being twins I got almost no sleep. But I would still say #1 was the hardest to adjust to. I had 4 under 3.5years so I don't think the age gap is necessarily the biggest factor. For me having them close was a major factor in having more so I just got on with it. I also think that not having another baby when you desperately want one and having to deal with that for a long time would be harder than adjusting to a 3rd baby.