My LO is 17 months, and yes around me there are people having second babies, so it does feel like I'm holding off somewhat and feel quite broody. But in reality a three or four year age gap isn't really a big one.
I also bf, and its funny because again I do feel like those who are on to their second babies are 'babying' their first ones a bit less, seeing them more as "big sister" or "big brother" and expecting a lot more of them. Mine has only just started saying words in the last month or two although he understands quite a bit more - he still feels very young. He needs me so much, I can't imagine having another baby to take care of right now.
I don't think I'm holding DS back in anyway though. He'll have plenty of his childhood not breastfeeding ahead of him for a start. I'd like to parent his siblings pretty much as I have parented him (as much as is possible anyway), so I'd like for him to be able to be occupied with an activity either at home or at the park for a while while I'm breastfeeding the baby, he's just not ready for that yet, I'm still having to run after him every five or ten minutes.
There's so much to think of when planning an age gap: personalities, fertility, breastfeeding, parental ages, lifestyle, childcare costs, etc. And then there are things you can't plan for - taking longer to concieve than you thought, miscarriage, contraceptive failure, the personality and needs of the new baby etc.
I think an 18 month age gap has worked okay for my Dsis, her older DS is reasonably verbal and sensible, her younger DS is very laid back and easy going. She's happy to rely on family for help with childcare, and as she works her first DS was already used to mummy being busy or away doing other things. But I just can't imagine my DS being ready for a sibling yet, and I'd rather not leave DS with family reguarly yet.
But then if we had had a surprise baby then we would have managed I guess. We don't want to leave it too long to TTC because it took a long time to concieve DS (part of the reason that I'm savouring every stage with him I guess, I had time to really long for a baby, and know that he may end up being an only not by choice) and because of DH's age. But there are reasons to wait too - my fitness, needing a bigger home etc. It's never going to be a perfect time to try either.
As always, do what's right for your family. I came from a big one with lots of different age gaps, there are positives and negatives to each and the quality of relationships is affected by much more than the age gap. Like you, I think the more lesuirely approach suits us and our little one.