@common, – and rasha and she eino yodea get the same posuk as the answer .. they are clearly related (the source escapes me for a second, after all the cups).
she eino yodea is treated arguably harsher than rasha. “opened up” v. “blunting teeth”? We tend to see the first as clearly figurative and second almost as literal. But if you look at them fairly – literally “blunting teeth” is probably a minor dental procedure while “opening up” sounds more like an invasive surgery… Furthermore, rasha is given a shocking answer that might lead him to a teshuva, while she eino yodea gets the same posuk, possibly with the same meaning, but he is not even shocked …
there are lots of sympathetic views of the 4th son relating to people with no background, etc and they are great, of course – but peshat is: he is sitting with his learned father – he aint an orphan! – and still has no idea – or no interest – how to ask a question, despite all the tricks at the table, eating potatoes in a grotesque way, not counting days spent at school … what is wrong with him or his parents/educators?!