My son will be almost 5 when the baby is born I think it's a bit of a big gap, but on the other hand, not really, especially when you look at the future. Soon, the big brother can keep an eye on his little sister when going out, and when playing outside, there's always a big brother around. I had a similar age difference with my sister and brothers (12 years, 3 years, and 1.5 years), so I basically had the same experience as my daughter had in her early years. I was always well looked after, no matter what.
I also think it's a nice idea that you can enjoy both stages and give them equal attention. I'll have the baby/toddler/preschooler phase, while my son is going through a completely different stage. I find that quite nice