I was one of two and I felt very lonely growing up, my sister was 3 years older, and we were and still are like chalk and cheese. She gets her opinions from the DM, me from mumsnet! She was into fashion, pop music, I was a book worm etc. As adults we get on ok, but if she wasn't my sister we wouldn't have got past the smiling at each other in a toddler group stage.
We have 3 children, and yes sometimes one does get left out, but more often all three tare around the house together. The older two are girls, but personality wise are opposites, ds is in the middle. There is 4.5 years between oldest and youngest, the oldest girl dotes on ds, whereas dd2 is more of a peer to both of them. If we didn't have ds I'm guessing that there would be more fighting between the girls, as it is ds makes their relationship less intense.
We aren't planning to have 4 due to finance, space (car and house), time, ability to let them do all the activities that they do, but also I don't think in our situation it would help much, the youngest one would be much younger - almost the same difference as between dd1 and ds, if they were a girl then ds would be genderwise even more outnumbered. There is no guarantee that they would fit in better or worse, our family at the moment seems to work, what if the youngest one was the one who always felt left out. Dd1 might drop ds and coo over the baby so ds would probably have to align to dd2. I know it works for some families, I guess only you know the personalities of your children and how they work as a team at the moment.