Thermal, and others, re,
In some parts of the world that would be an augmented crew.
In others the complement of an augmented crew would be 2 Capts, 1 F/O and 2 F/Es.
The rules I operate under would allow 16 hours duty.
Unless your airline is based in Canada, where a crew of 3, (C, F/O, Augment) can operate for 20hrs if there's a "legal" bunk, and 23hrs in "unforseen circumstances".
Now those are the CARS - individual contracts, for which pilot associations have handed over negotiating dollars to obtain a standard higher than the Canadian Regulations, permit 15hrs with 3 and 18hrs with 4 - 1 Captain, 1 F/O, 1 RP and 1 Augment F/O according to one contract - the complement varies but not the attitude.