I have no Kids
BUT I am the oldest of 7 and I have 3 cousions who I grew up with who are 1.5yrs apart or less. So I can give you some of what it feels like to have siblings at different ages.
I would personally do 4 years, unless the first child is mostly independant.
I was 3 and already fairly indepedant when my first baby sister was born, it worked out but it really depends on the kid. Father says that at first I did not care for her too much, but I got over it and love her to death now.
My baby brother (and other younger siblings were bron) was born when I was...oooo...around 7? That worked out much better for me, I understood what was going on and was quite interested and willing in the whole thing. I started out loving him! vs. thinking he was bad like I did with my sister for her first few months.
My 3 male cousions, they worked out in the end, but it was quite stressful while it was going on. Now they are the best of friends as preteen/teen boys could be.
Suprisingly they do not fight to much, and they do make up quickly and they can talk about similar school and life stuff.
That I could never do with any of my siblings(different life stages and all, only advice and experiance I could give). I could have possibly with my my first baby sister but she has Lissenesephely(sp?) (Smooth Brain Disorder), so I do have a slight info/bonding gap(err gaping hole) there with her.
It is neat seeing a baby grow up when your a middle aged/younger kid to be honest. It is like babysitting without much responsibility! When you are more of a preteen/teen it turnes into just babysitting, still fun but less magic and wonder involved.
also...sorry for bad spelling, IE has no built in spell checker and boyfriend doesn't want me downloading stuff on his computer. Like I would break it, I am the one who fixes the darn things for a living!
I should add, while growing up I liked the 7year gap more (little brothers(2, different moms), youngest cousin). The 5 yr (oldest cousin), we are super close, but not until later on is my 2nd favorive gap(close with the 7yr). The 3 yr, I cannot say for sure due to her medical problems, that lead to confusion until I was able to understand it better. The 10yr with my little sister was alright, I do think of her more as a kid/fathers other spawn) most of the time, then a little sister. That gap might have been to big for us. I do love her, took until she was able to talk at me and be my friend for me to see her as a sister in my mind.