In my experience as well, Sonnet 4 is much better than Opus. Opus is great at the start of a project, where you would need to plan things, structure the project, figure out how to execute but it cannot beat Sonnet is actually executing it. It is also a lot cheaper.
Opus is too expensive and I find it goes way off the rails often (just writing way way too much. Maybe that could be controlled with a better prompt on my end). Sonnet gives me more realistic code that isn't too overengineered.
I've been going down to sonnet for coding over opus. maybe i am just writing dumb code