How much of the 4tb drive has been used?
How much remains free?
What I might do in your position:
1. Defrag the 4tb drive so that all the files are grouped "towards the top" of the drive (i.e., "at the beginning", with the rest of the space "free").
2. Partition the drive (how large the first and second partition will be depends (again) on how much space is currently being used on the drive. This will result in the "first" partition having all the extant files, and the second partition "open and free".
3. Next, MOVE all the files from the first partition to the second partition. You want the FIRST partition to become your OS install.
4. Now, clone the OS to the first partition (which is now able to accept it).
If the video files are important to you, I would suggest you get a SECOND drive, and back them up to it....
You can do this without any problems with or without partioning the external hard disk as long as there is enough space on it. I have done this several times, and instead of partitioning the disk I simply put everything that was an that disk into one folder - just to make it "cleaner" looking after the cloning.