I won't lie, I'd like to see a new spec for rogues, like a ranger/sniper spec, just so there's some interesting spec different from the others, I don't think 4th specs will happen, at least any time soon. Blizzard has been doing a good job improving the classes/specs, you can definitely say things have improved since vanilla. For a long time blizzard has focused on balancing the 3 specs with each other other classes specs. Adding a 4th would change a lot and be a lot harder to balance. Besides not all specs need or have a 4th spec. Druids had both bear and cat in the feral spec before, a lot of talents and the basics for both were all there, they just split it in mop. No other class has a 4th spec like that. Some specs people think up don't make sense, because those classes don't need another spec. Also, GC mentioned another thing keeping them from changing classes with all dps specs is because people don't want to be a hybrid. He said, on his twitter, that more people leave from changing classes then from neglecting them.
I don't think 4th specs would be a good idea.