This is so far off my experience. Grok 4 fast is straight trash, it literally isn’t even close to decent code for what I tried. Meanwhile Sonnet is miles better - but even still, Opus while I guess technically being only slightly better, in practice is so much better that I find it hard to use Sonnet at all.

Not Grok 4, the code variant of Grok. I think it's different - I agree with you Grok 4 kind of sucks.