My boys are 18 months 4 days apart, I love it it has its days when I think why oh why did I have them so close together, but then they play so well together at times!!

I dont think there is any right or wrong time, its what you end up having you will never know any different IYKWIM.

If you have them close you have 2 in nappies, possibly tandem BF, a toddler that demands attention as not old enough to do things for themselves.
You have a 4 + age gap you have a child who can get jealous, a baby who is often disturbed to do school or kinder pickup/dropoff, but you have a child who can help you too a degree.

Good luck