I guess it kinda depends on your surroundings. If I’ve forgotten my phone then most likely I’m out and about and dictation doesn’t really work there.
It works nicely for me outside of my home (not that home is a quiet place for me... my toddler and baby seem to hate peace and quiet, and have teamed up to do everything in their power to ensure we have as little of it as possible). I don't recall if there was a big difference in going from a Series 1 to the Series 4, but the number of places I can get away with accurate dictation is surprising to me. But then it's worth noting that dictation is near-perfect for me in quiet locations, so I'm already coming from a solid performance. I know dictation fails some people even under perfect conditions, so it makes sense that adding background noise won't help matters.
What I'd hope others take way from our exchange is that dictation
can work really well, to the point of replacing texting on your phone (assuming little to no esoteric language usage or specialized terminology)... it just depends on how well the dictation feature works for you. Evidently, it can range from fantastic to unusable.