If a pilot deviates from an ATC clearance in response to a TCAS Resolution Advisory (RA), what is the pilot's primary communication obligation?

1. Notify ATC as soon as possible after commencing the deviation and advise when returning to the clearance
2. Obtain ATC clearance before responding to the RA
3. No notification is required as ATC receives the warning automatically via radar
4. Notify ATC only after the flight has completed and landed