The first step is to set your video mode to the final resolution you want for the output, so 3840x2160.
The Crop:Source filter cannot be keyframmable (animated), so I recommend another filter.
Side note: you can still use the Crop filter if you do not need smooth animation (as in: after 1 minute you suddenly move the crop area lower/higer in one single step by splitting the original video into multiple pieces and adding individual Crop:source to each split subclip).
The filter I would use is “Size, Position & Rotate” filter. Fill out the resolution to 3840x2880 inside the filter - this will “zoom in” to the video and top and bottom will be outside of view.
You can now adjust vertical position to move the video up and down so you have your subject in view. You can use keyframes for smooth movement.