I am a lot younger than my siblings by 7 years. And I am the fifth child.
It was hard at times growing up because they were able to do so much more than me. At the time I didn't understand why they could stay up late, etc.
I don't feel that I had a lonely childhood. My sisters and brother did include me in some things. Which, I am sure my parents made them!
Now, we are all very close. I love hanging out with them and I look to them for a lot of advice.
It is nice that I was around when they were having children. I have been around kids almost my entire life. Now that I have a family, I feel so much more comfortable with a child. And now my nieces and nephews can enjoy my daughter and they can baby sit.
I think if you want to have another child, don't do it because others think you should or so your youngest won't be lonely. He will have lots of attention from his older brothers.