If 50GB is too large, use HandBrake (free software). Convert the file to a smaller size (10-15GB) using the "H.265 10-bit" preset. This retains nearly all of the visual quality while cutting the size by 75%.
Most major digital retailers allow you to purchase or rent 4K content, though offline download availability for true 4K resolution can vary by device and platform. Download 4k Uhd Movies