@dobey, you are right about 4.0 version, it doesn't exist according to the provided specs. Seller mixed up the info. Finally, I purchased two items. One with Android 6.0.0 and unlocked bootloader, and another with Android 6.0.1 but locked bootloader. So I don't need to flash Android firmware first.
Installation to 6.0.0 was smooth and done in five minutes! This is awesome!
Installation to 6.0.1 took an half of an hour due to locked bootloader, and Ubuntu didn't start after the first attempt of flashing. It was Google logo on the screen and nothing happened. Fortunately, the second attempt was successful and also done in five minutes! Thanks to all of you, guys, all developers and
ubports-installer-0.4.11-beta that also helped to unlock a bootloader in easy way!