Framerate was prioritized in this case, which is not something I can take much issue with.
The PS4 Pro version of Hellblade will in fact have several visual and technological improvements over the vanilla version- and now, we have a better idea of what to expect from the updates that the game will have when played on the more powerful PS4 console.
A post on the PlayStation Blog announced that the game, in an effort to maintain a constant framerate (which, to be fair, is paramount in a game of this nature, so more power to Ninja Theory for prioritizing it in this case), will adjust resolution between 1080p and 1440p dynamically and on the fly. So it seems that not only does the PS4 Pro version not hit 4K, it does not even maintain a 1440p resolution at all times.
Which, like I said, is fair- framerate is the most important thing in a game like this, and I am glad to see it was emphasized in this case. But for people who were hoping for 4K performance for the game on the PS4 Pro, this news may come off as a bit disappointing.