This tweet is worded weirdly, it's native 4K as confirmed by Digital Foundry. The 60fps mode sounds like Dynamic 4K.
As listed on the updated game page, Ratchet & Clank: Rift Apart is displayed in “dynamic 4K and HDR” instead of native 4K resolution. This implies that the game dynamically changes between resolutions depending on what is happening on the screen in order to preserve visual quality and framerate.