So yesterday I was doing some research and found out that Blizzard recovered the domain diablo4.com in January 2019.
You can check the whois history at WhoisRequest or the screenshot I took and the whois details
The domain has been on parking (on sale) since it was created in 2002 and only now Blizzard has claimed its ownership. This makes us closer to a release and makes the game more likely to be named Diablo 4.
To give you some perspective, the domain diablo3.com (which is now Diablofans) was claimed only one month before the announcement of Diablo 3 at the WWI in Paris.