There is a lot of armor to unlock in Halo 4. Most of the Halo 4 armor sets don’t include armor for Forearms, so you have to unlock them separately.
Here is the list I have compiled of all the forearm armor I’ve seen in all of the dev videos. I think I have them all, but let me know in the comments if you find anything that’s missing.
Unlockable Forearm Armor
All of these can be unlocked naturally in the game.
Contoured – Reach rank SR-39
Contoured CHVR – Unlocked with the commendation Perfection – Master
GV-09 Locking – Reach rank SR-22
Inner-plated – Reach rank SR-30
Mark VI – Complete the Campaign on Legendary difficulty (solo or co-op)
Outer-plated – Reach rank SR-34
Twin-plated – Reach rank SR-9
Twin-plated AEON – Unlocked with the commendation Close Quarters – Master
XV-27 Shifting – Reach rank SR-48
Exclusive Forearm Armor
Here is the list of forearm armor that is only available from pre-orders, collectors editions, or console bundles.
Fotus – Included with Halo 4 console bundle
GV-09 Locking FRST – Pre-order bonus Gamestop
Inner-plated WEB – Pre-order bonus from Amazon
Outer-plated RPTR – Pre-order bonus from Best Buy
Recruit PRME – Included in the Collectors Edition of Halo 4
XV-27 Shifting CRCT – Pre-order bonus from Walmart
XV-27 Shifting PULS – Pre-order bonus from the Microsoft Store