I got it in campaign, it gives me four items instead of three. I just wanted to know is that the only perk? Or does it have better chance of giving me a legendary, perhaps even a mythical?

Only Gold boxes on the map give mythicals. The Red Gold box at end gives best chance for mythical I believe! I opened mine and receives 4 Ultra Power kits and 1 Mythical Meteor. The npc bots could not handle my heat mech.

Woah

Keys? How do you get more?

Tokens of course.

There are boxes with 2 chevrons which give 5 items as well.

This is the only difference from normal item boxes.