How do I unlock all 4 accessory spots?

[oHAMBURGLARo]

I just started Chapter 12 and I still only have 3 accessory spots for each character. When and how do I get the 4th?

[tiornys]

For this one, you do have to beat the final bosses. That unlocks the 10th stage of the Crystarium, which contains each character's 4th Accessory slots (and the 5th role levels, and a ton of +HP).