Sony recently confirmed that they are investing money in OLED displays. They will become the first company which will release smartphones with 4K OLED screens, and this information was confirmed by Hitoshi Osawa, Sony Mobile’s Head of UX product. What’s more, it may happen very fast because the newest phone from Xperia series - Sony Xperia XZ Pro - may debut with such a display.
There is a chance that Sony will present this phone on MWC 2018, but these chances aren't too big. Basing on previous rumors we can say that Sony Xperia XZ Pro may have 5,7-inch display, Qualcomm Snapdragon 845 processor, 6GB of RAM, 128GB of internal storage and dual camera on back.