No, the camera sensor resolution isn't the only factor that determines whether a phone can shoot high-res video, regardless of the APIs offered by the OS. Phones have special video encode hardware to make a video file from the raw camera data, and the hardware can only encode so many pixels per second. To record 2k or 4k video, the video encode hardware has to be fast enough, and other parts of the system need enough bandwidth/throughput to handle the data rate.
As for Google Camera, there's no point speculating about its future releases. They might choose to focus on a new feature instead, or may wait until more phone hardware supports 4k video. They might even include it in the next Nexus phone but wait another six months or so before releasing the feature more widely.