I doubt they'd do it because they're charging money per retainer right now, but I'd absolutely love if they did. I spent like two hours cleaning and organizing my retainers the other day, purely because I quite literally ran completely out of inventory space while leveling my crafters. As in I had three full retainers (I'm already paying for one extra), a full chocobo saddlebag, AND a full inventory. The process made worse, of course, by the retainer menu being so clunky to use... but I digress.

Having a fourth retainer would be a huge help. The only reason I'm good with paying for a third is because I'm paying the reduced sub, so with one retainer I'm paying as much as I'd be paying for WoW or full access in SWTOR :P and boy does space still get tight sometimes