For what it’s worth, that doesn’t sound typical to me. I’d say pre-school, it’s it’s done at all, is done at the age of 4 - maybe 3 if you’re doing it super early. I’ve never heard of “K4” and “K5,” and most kids start kindergarten at age 5 and only do one year of it.

Where do you live?

In the US, kindergarten starts at 5/6.

A note on this: We used this book to teach our two oldest kids to read before they entered kindergarten. Kid #1 was asking to learn to read at age 3.5, and that's when we got the book. She could really read before her 4th birthday. It was indeed amazing. We then started trying to push it on kid #2 at age 4.5, since our expectations were set by kid #1, and we now regard this as a mistake. Nobody was having fun, so we ended up setting it aside for half a year. We picked it up again in the summer before kid #2 started kindergarten, and she finished learning to read with ease at that time. I think we're lucky that the initial attempt didn't ruin reading for her. We're being much more gentle about introducing it to kid #3.

Kid #1 was and still is an oddball in a number of ways. Learning to read so early seems to have given her some advantages, like being able to consume volumes of information at a young age, but she'll have a harder time in other ways.