No plans for 4th jobs - I don't see the point of it. I would much rather create brand new classes than rehash the existing ones because there is only so much you can do before the classes start changing so much they are like different classes anyway - if you don't do that, you just end up with the same thing as before with slightly more powerful versions of the skills - waste of time tbh.
Even creating one new class is gonna be a massive job since we have 3 jobs now. I am considering making some new ones that just have one job and can only be created if you already have a max level 3rd job character - like a hero class, but it's only an idea at this stage.