I hadn't heard the 4? -->1 explanation, but unsurprising that such a meme would circulate
But your source gets a (US) D, if Æ really did get '4's: 1=='sehr gut' ie mucho bueno.
http://www.justlande...ion/State-schools

And it wouldn't surprise me if he Got '4's: it must have been unutterably boring for him, reading Heinrich und Jane in such a er, wissenschafft (Not!)

I think I can find some authentic examples though. And Summerhill (UK) -- an early experiment in non-didactic 'teaching' -- is replete with examples
... like the kid who hadn't learned to read at usual appropriate time. One day asked a Teach about butterflies, about which he had become Interested. He replied,
"Sorry, know little ... but we have several books on them in library.."

Yep, you guessed it ... IIRC a couple? few weeks, in any case. Probability of truthiness: est 0.9 or so.

:-0