Samsung confirmed in its Analyst Days presentation slides a while ago that it has a Ultra HD 2160 x 3840 pixels Super AMOLED displays for mobile in the works for 2015, as you can see above. Also, at a Semiconductor and Display Technology Roadmap workshop last year, Samsung presenters hinted again at having higher-resolution UHD smartphone displays in the pipeline. These resolutions, however, were mentioned then together with the "diamond pixel" term, which is a fancy way to say a non-standard pixel matrix arrangement, that is closer to PenTile, than the standard RGB stripe matrix. In any case, at 700+ ppi, nobody would notice the individual pixels anyway.
Furthermore, we received a word that Samsung will be putting those UHD Super AMOLED panels in none other than the Galaxy Note 5, and will start the displays' production in August, which would jibe with an eventual September announcement around the IFA 2015 expo. The exact diagonals are said to be 5.78" for the inevitable dual edge Note 5, and 5.89 inches for the regular, flat display version. These return exactly 762ppi and 748ppi, respectively, or the highest pixel densities ever attempted on a phone, if this Note 5 speculation pans out. The more, the merrier.