OP I think it depends on the kid, but I'd err on the side of 4 being a little bit too young. My DS1 went into a top bunk when he was about 5 I think, but he is over the top sensible, so I didn't have any concerns with him. I had more concern with the 2yo trying to climb up, though after I made it clear she wasn't allowed up there, that wasn't a problem either.
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