I had my twins six years after their brother. I do see a lot of exclusion with them. They love their older brother, but he's busy doing teen things and they have each other.
Now, my parents had three girls; my older sister and I are 2 yeas apart. We played together when we were young and fought when we were teens (typical sibling rivalry - especially when it came to the bathroom, we only had one), and as adults we keep in touch regularly. 13 years later along came my little sister. We loved her and took her with us, but we used to tease my parents they waited on having kids until they had built in babysitters.
My little sister was just starting Kindergarten when I was a Senior. After I graduated we didn't see each other very often. Now as adults, we don't talk as much. Not because we don't like each other, we do love each other and I'm the family mediator when she and my older sister bicker, but because we don't have much in common. Our taste in music is night and day, the way we run our households is well, galaxies apart, LOL. But I tell you, when the whole family reunites we complement each other very well. And it's never boring.
So, it works, but I do wish I knew my little sister as well as my older sister.
I do see my teen son (he's 16 now) keeping tabs on his brothers and he'll mention things they like (he even helps me Christmas shop for them). And the twins (who are 9 - they'll be 10 soon) keep tabs on their brother, so it's nice to know that they care enough to notice little things.
That didn't help at all, did it? I guess age difference doesn't really matter. It's what you and your husband are comfortable with.