4K video of the Mercury Transit is available.

This sentence refers to a phenomenon called the "Mercury Transit" captured in a 4K video format. A "Mercury Transit" occurs when the planet Mercury passes directly between the Earth and the Sun, appearing as a small dark dot moving across the Sun's surface from our perspective. "4K" refers to the video resolution, indicating that the video is of high quality and has a resolution of approximately 4000 pixels horizontally. So, the sentence implies that there is a high-quality video recording of the event where Mercury crosses in front of the Sun, and this video is captured in 4K resolution.