It's just different. It's about becoming a parent again, iykwim. Up until the age of 5 or 6, you are constantly reaching milestones. Weaning, walking, talking, potty training, nursery, school, etc. All governed by how you parent. Then between 5 and 11, it's pretty straight forward, imho. Life seems to get easier. Life just ticks along nicely.
Then suddenly, they reach teenagers and you've forgotten how hard it was in the early days and you think it's far harder to be a parent of a teenager. I think probably, it's no harder, just different.
I am, right at this moment, having a teenage parent moment! It is tough, but I'm sure, by comparison, no tougher than being the parent of a two year old.
3 or 4, will make no difference. On the up side, by the time smallest is old enough to be out after dark, the oldest one will be old enough to pick her/him up from where ever they are! 