Go with 200 OK. The page exists, so technically it's not "gone". Too many 4xx errors might mess with SEO. Instead:

1. Use 200 OK
2. Show the job details with a "closed" message
3. Add a "noindex" meta tag
4. Maybe add a canonical link to your main job page
5. Use structured data to mark it as expired

This way, you're honest about the page existing, but you're telling search engines, "Hey, don't bother with this one anymore."