more or less, the Shared Folders aren't fully displayed (click the right hand 'v' buttons to show the details). I forgot to say welcome to the forum, welcome!
From the second screen shot we can confirm that your two hard drives are configured in RAID 1 (that's mirrored RAID). That gives about 3.6 TB of raw storage. But we have to remember that there is a DSM partition on every drive which takes a little bit of space, but also means that the basic DSM will still run if a NAS loses all but one hard drive.
So we now see that Storage Pool 1 has been configured with one volume, Volume 1, that's the usable storage space that you can use for your Shared Folders and installing extra packages. Volume 1 can be seen to have a
maximum available space of 3.5 TB, and you have used 665.2 GB of that with you upload of movies (and may be some other things) ... that's why you see that you have 2.83 TB of available storage left for adding more stuff.
Your first point was if you have installed 4 TB drives why do you only see 3.6 TB for each hard drive. This is due to marketing and then spineless committees relenting on what 1 KB, 1 MB, 1 GB, 1 TB, etc. means. Marketing people are cynical and gold plate things, so they see 1000 B = 1 KB, 1000 KB = 1 MB, 1000 MB = 1 GB, and 1000 GB = 1 TB. Whereas a computer working in base2 sees 1024 B = 1 KB, 1024 KB = 1 MB, 1024 MB = 1 GB, and 1024 GB = 1 TB. Unfortunately the standard bodies have relented and the official meanings are now using base10 (i.e. 1000 B = 1 KB) but no-one told computers. The result is forums spend a lot of time explaining this, and marketing types dupe buyers and get big cars.
So now your 4 'marketing base10' TB HD has 4,000,000,000,000 B of storage but the NAS is dividing this by 1024 x 1024 x 1024 x 1024 to make 3.638 'base2' TB.