This is a double digital release for Yen Press :
No Game No Life 4 is now available to be directly bought in ebook format ; Like other news, just follow the link in the news to go to the download page. (here amazon page)
I did say I will try to follow the digital releases from now and I will notice you when books will be release or available to pre-order... well on amazon anyway. Also if you know of other publisher that I could follow, other than Yen-Press, I will gladly take your suggestions.