Samsung just sent press invitation to their Unpacked 2014 Episode 2 event, which will take place on September 3rd. It's highly likely that the Galaxy Note 4 will be announced during the event (they even say "note the date").
As always before a major Samsung launch, we heard several reports (rumors?) regarding this device. Last month SamMobile claimed to have exclusive info detailing a QHD (2560x1440) AMOLED panel. On the same day, the Korea Times reported that Samsung will actually unveil two Note 4 variants - on with a regular flat display, and another with a curved one. A month earlier, a senior VP at Samsung Electronics said that the Note 4 will have a "new form factor".
My own personal bet, is that during this event Samsung will launch a single device, which will sport a flat (glass-based) QHD Super AMOLED display, probably 5.7" in size. This is a major launch for Samsung and they do not have the capacity to adopt a flexible OLED yet. But it's likely that they will later on release a variant of this phone with a curved OLED, maybe in a different design than the Galaxy Round, and maybe this time it will also launch globally.
The Note 3, by the way, had a 5.7" Full-HD Super AMOLED display (386 PPI, PenTile). Samsung launched several variants of this phone, including the Galaxy Round - the company's first flexible OLED product. The Round had a similar display (5.7", Full-HD) - but one that is curved horizontally.