Here in Ireland a lot of kids (approx. 50%) start pre-school at age 3 and formally start our primary education at age 4. Whats the big deal
In a few (don't know about most) European countries the one pre-school year starts at age 5/6 and first grade at 6/7.
At which age you start depends on which part of the year you were born (so you don't start too early or too late).
"Pre-schooler" is also in some languages used exclusively for kids starting actual school next summer.
Kids younger than that are simply "kindergarteners", not actual "pupils".
A few kids start a bit earlier than average, but I don't think that happens very often.
Pre-school is usually an hour or so in Kindergarten/nursery school, when they teach pre-schoolers specific stuff to assess understanding and prep them for school. Other than that it's just nursery/Kindergarten and you mainly play and sing all day long.
Kindergarten/nursery is rarely called "school", because school is associated more with academia and academic learning.
I always find it weird when people say their kid goes to "school" and they're like 3 or 4... In my head I always think "no! your kid doesn't go to school! But to Kindergarten/nursery (school! lol)! It's not the same!"
I honestly dislike the term "school" for toddler age incarceration (lol sorry, couldn't resist!).
Kindergarten is a German word and literally means "children's garden", which is a far more fitting term in my experience for that age!
And that same type of word is used in other languages as well.
What does primary education even look like for 4 year olds? I can't grasp that concept, I can't imagine it's any different to other countries' singing, playing, arts & crafts and story times. Writing, reading, maths etc all start in 1st grade at age 6/7 and that's a slow-ish introduction.
Anyway, I'd say it's mostly how different countries use language, like "school" starts with 1st grade (age 6/7) and anything before that isn't seen as "school".