The author of the text (which is in German) is purportedly a former member of the Islamic State who wants to warn the West that ISIS possesses four nuclear devices, and plans to use them against the United States, Russia, France, and Germany. What he says may well be a hoax, or more specifically, deliberately planted disinformation to achieve particular political ends. I can think of several Great Powers who would find it useful to disseminate the fiction that ISIS is ready to use nuclear weapons.
However, it could also be true. If it is, you may be sure that the intelligence services of Russia and the major Western powers are well aware of it. As the ISIS defector points out, the West has moles inside the highest levels of the Caliphate.