Daniel M. Russell poses what he claims is a deceptively simple brain teaser in his blog:
What short 4-word idiomatic phrase (in English) captures [the] idea of a problem that seems impossible, but actually has a simple and obvious solution?
"Deceptively simple brain teaser" is the best I could come up with, but I don't think it's particularly idiomatic.
So, the originating blogger has posted his intended answer. I would agree that there was no stand-out candidate amongst all the plausible suggestions, so kudos to all those who found a solution.