Longhaul Crew on board question

[ct976]

there are 4 crew on board for a long haul CX flight according to the website on the 744, 346 or 343

what is the crew? how many operate at a time?

leftseat: Captain
rightseat: First Officer
backseat: Second Officer
4th: ????

or is it just a rotation like:
during critical phases,
left seat: Captain
right seat: Senior F/O

during cruise phase,
left seat: F/O
right seat: S/O

[dragon501]

It works with a 'relief commander' which is the full 3 bar fo on the crew.