http://www.ontarioimmigration.ca/en/pnp/OI_PNPNEW.html1) Minimum CRS 400 is old mandate, it has changed a week back.
2) Work contracts means anything related to work like salary slip, experience letter, letter of appointment, promotion letter(its required as a proof that you are actually doing a job)
3) & 4) what do you have in job offer and relieving letter, read it carfully, does it state what kind of work you do, your role and its activities in detail?
see this link for info, read it carefully, it would make you clear that why you were chosen for NOI