Thank you, Brandon. I agree, that my English is very basic and it might be difficult to understand what I wrote. For example, “…what is a maximum allowed INTERNAL temperature…”.
Any way, I have read all information you wrote me as an answer, before asking my question. I mean, my product overview as well, as a really great WD Answer ID 1704.
All what you wrote, might be said in the following very simple words: “I do not know and I do not want you to know”.
So, based on a previous answers from WD (22/10/2014 08:57) regarding the same question, but about my old My Book devices, I can answer by myself as follows.
Inside my new My Book 4 TB device might one of the two following hard disk drives (PDF downloaded):
WD Blue 4TB Desktop hard drive with Environmental temperature 0 – 60 Celsius
WD Black 4TB Desktop hard drive with Environmental temperature 5 – 55 Celsius
Your support answer then was: “WD Internal hard drives maximum internal temperature is equal to the maximum environmental operating temperature for this disk”
Thus, maximum allowed INTERNAL temperature for my new My Book device might be 55 Celsius.