I don't understand how people can fill up even 3 retainers let alone 4. I for one do not have a problem with inventory space, we get so much of it as it is and I do a heck of a lot of crafting. It's surprising how much junk some people hold onto, a stack of 5 iron ore which can be purchased from an NPC at any time for peanuts, a stack of 20 kukuru beans that costs 15gil each on the MB, then there's the small number of people that have 2-4 glamor sets for all 15+ jobs, why oh why?!

Recently, I do not recall where, it was stated that the game backs up every 15 seconds to prevent rollbacks if a server crashes, and that by increasing inventory size they risk increasing the 15 second backup time to the point where a crash can result in a rollback (and lost items, quests having to be redone etcetera). With that in mind, I would find it highly unlikely that they'd give everyone a free 3rd retainer and or increase inventory space to the amount the minority of players seem to need for their hoarding.