Wow! All my life I heard my teachers saying open the book on page x. And I say that to my students too. My jaw is dropped.
But from now on I'll use at or to.
I've got the same problem.
My teacher always said "open your books
on page 4"
When I came across "open your book
at page 4" in Teacher's book published by Longman I was sure that's a mistake.
I looked it up in Longman Exams Dictionary I found:
"(on) page 5/20/360 etc
▪ The address is given on page 15."
However, now I can see that there is also:
"see/turn to/go to page 22/49 etc"
So what's the rule?
when do you use "on" and when "to" with the word 'page'?
Is "open on page 4" only unnatural or is it a mistake?
I see that native speaker wouldn't say "open your book
on page 4"