Will there ever be 4 TB (as a single drive) or 6 TB portable hard drives ?
Yes, but not soon. 9.5mm 2TB is still a new thing, so you'll have to wait.
I've had as good luck with manufacturers as 3rd parties, and 3rd parties often have useful case features (like bumpers, integrated cables, cable holders on the case, a carrying case that can fit the cable(s), etc.). I treat mine well, and have no issues, and the two other people I know that don't abuse them don't have much in the way of failures, either. Small sample size, I have a feeling the cheap cases and regular user abuse has a lot to do with the common failures of externals, especially given how I see them treated when I'm at a coffee shop or sandwich joint.