You also gain tier 4 potentials, which is where you'll see 10% unique lines and 13% legendary lines as opposed to tier 3's 9% unique and 12% legendary.
Can you explain that "tier" concept? It's a new one to me (although I did notice higher %ages on alien equips
Taken from here. Note that, while it specifically states "for weapons", the same applies to all equip items; armor, accessory and weapon. (less)
