Another one here who can definitely recommend four. I always wanted four and, although I think I'll be broody for life, I can honestly say I don't want anymore now. It's such a lovely feeling to not be consumed with wondering about having another (especially as DH always took some convincing, so I was never positive we'd end up with 4).
DC4 has just turned 1. I wouldn't say I haven't noticed the extra work. She's not quite as placid as DC3 and took longer to settle into a nighttime routine than my other 3. But she is a delight and, of course, none of us would be without her.
Part of my desire to have 4 was the everyone-having-someone-to-play-with thing. I was one of three myself. And I must say that that is working out brilliantly. The kids often play in pairs, and it's not always the same pairs, which is just great.
My gaps are a bit bigger than yours. When DC4 was born, the others were 6.9, 4.11 and 2.8. I agree that it'll be tough for you with smaller gaps, but then again, I can see your DH's point of view about them all being at the same stage together. Even with small gaps, if you have 4, the oldest is inevitably going to be into different things than the youngest. But at least you are minimising that problem so things like holiday choice, days out etc will be easier in that respect.
Enough rambling from me. I think if you are anything like me, and thoughts of another baby just occupy all your time, then go for it. It'll be hard work. But you won't grow old with that regret and what if? feeling.