Mixed Armour on page 76 of 4thEd rules, big box labeled "Mixed Armour".
Also, take a look at the "Torrent of Fire" rule, page 26, right column, third paragraph (the long one), line 6, "Sometimes, however, a unit will be subjected to a torrent of fire so severe that..."
Since these usually are a bit harder to grasp, I'll make an example:
5 PAGKs (3+), 1 Justicar (2+)
1 Plasma wound, 7 Bolter wounds (the to-wound process isn't important for these rules)
Since You have inflicted more wounds than there are models in the unit, You get to nominate one model to take a save. You nominate the Justicar, and the opponent chooses a Bolter wound to save against. ("Torrent of Fire"). 1 Plasma, 6 Bolter wounds remaining.
Now the Mixed Armour kicks in, and 5 wounds are assigned to the Majority Armour save of the PAGKs. Opponent takes 1 Plasma wound and 4 Bolter wounds on the PAGK, 2 Bolter wounds remaining.
1 Bolter wound is allocated to the Justicar.
The remaining Bolter wound wraps back to the majority Armour of the PAGK.
Result:
Justicar saves against 2 Bolter wounds.
PAGK save against 1 Plasma, 5 Bolter wounds.
Note that due to "Mixed Armour", the wounds taken by Justicar can only be inflicted on the Justicar, and those taken by PAGK can only be taken on the PAGK. Examples:
Justicar fails both of his saves, resulting in two unsaved wounds. The first wound kills the Justicar and the second is discarded, since there are no other models with the same Armour save to remove the casualties from.
The same applies if all of the PAGK failed their saves: The unsaved wounds could only be removed from models with the same Armour save as the PAGK, and thus can't be dealt to the Justicar with a different save.
If the Justicar had the same save, the You wouldn't use the "Mixed Armour" rules at all, but the normal Wound allocation process... Of course, since there isn't Mixed Armour to begin with!
Note on the "Torrent of Fire" rule: Even if the Justicar had the same Armour save as the rest of the unit, if it fails against a nominated wound, the Justicar is removed as a casualty.