GPT4 is better at responding to malformed, uninformative or poorly structured prompts. If you don't structure large prompts intelligently Claude can get confused about what you're asking for. That being said, with well formed prompts, Claude Opus tends to produce better output than GPT4. Claude is also more flexible and will provide longer answers, while ChatGPT/GPT4 tend to always sort of sound like themselves and produce short "stereotypical" answers.