My twins will be 3 years, 9 months old when this baby comes (this one was actually conceived 2 days after their 3rd birthday). It is a further spacing than I thought I wanted, but now I'm really glad for it. Our situation is somewhat different since the first two kids are twins. I think it can be really hard for a child to be an only child for close to 4 years, and then suddenly have a baby sibling. I'm hoping that since my kids are already used to sharing our attention, there won't be quite as much jealousy as there would be were this to be their first sibling experience.
I initially thought I'd like to have my kids two years apart. That absolutely wasn't going to happen since with nursing twins, I didn't get ppaf until they were 21 months old. Then I thought an ideal spacing would be 3 years apart, but at the point when I would have had to start trying (when the boys were 27 months), they were both still nursing very frequently, sleeping in our bed, not sleeping through the night, and generally going through a hard time with their relationship to each other. Dw and I decided that we would start trying as soon as they were mostly weaned (down to 2-3 nursings a day), out of diapers, and sleeping in their own beds most of the time. All of these milestones were met by the time they were 2.5, and we started trying a couple months later. It took five months to get pregnant, so that's how we came to our spacing. Since I got pregnant, the boys weaned completely (nursing is really painful for me during pregnancy, and didn't feel right at all), and I'm glad I wasn't pregnant earlier for that reason alone. I'm glad they got 3 years of unhindered nursing (not that there wasn't some mama-led weaning in there, there definitely was). They also *stopped* sleeping through the night, and are now back in our bed most of the time, but since they're not nursing anymore, I appreciate the extra snuggle time and don't mind at all (although I do wonder how it will work when the baby comes).
I hope that my boys won't be too much older than this baby to form a close relationship with him/her. Ideally, I'd like to adopt a 4th baby when baby #3 is 2, but I don't know if I'll be able to get dw on board with that plan (she's thinking 3 is enough). I do feel sort of bad for this baby to not have a built-in playmate the way my twins do.
Lex