Japan preschool from 4 years old is allowed by schools?

My wife is the protective type for our children. And she is scared of bad things happenings to schools especially to younger kids. We didn’t opt in to daycare (保育園) because she thinks it’s too young to detach kids to their mothers.

I am aware that preschools (幼稚園) starts from like when they’re 3. But my wife wants to delay it one year until our son is properly potty trained at home, can speak what they feel, etc.

We are doing the homeschooling at home like independency, clean up toys, eating by himself, things thought in preschool already. We socialize to people with kids too and from our church as well. Question, does Japan allow that kids enroll from when they’re 4 and skip the 1st year as long as the kid passes all checks?

TYIA!

by genkiganko