If you've never played the Diablo series prior to now, this is not the best place to begin. I'd recommend Diablo III instead, which you can actually try for free on PC. For D2R Items players, Immortal is at least worth a try, though be aware that this game is demanding of your time or, preferably it will take a significant amount of your money.
The Diablo franchise is in a strange state in the present. Diablo Immortal has been a huge revenue-generating success for Blizzard however, the game has damaged Diablo's reputation because of its controversial, perhaps predatory way of monetizing. Diablo 4 has looked impressive in gameplay reveal trailers and the like, though some fans have expressed concerns that it could duplicate Microtransactions from Diablo Immortal. Blizzard has attempted to calm players about the microtransactions in Diablo 4, but we won't be sure of how it will work until the game is officially out.
Diablo 4 currently lacks a time frame for release, but it's expected to be released at some point during the first second quarter of 2023. It's looking like Diablo 4 fans who are interested in knowing when they'll be able to play Diablo 4 should be sure to tune in to The Game Awards in December.
Diablo 4's world can allow players to go off and begin the story whenever they want, but while this does "allow flexibility for non-linearity" The story will have "a beginning, middle, and end".
Through an interview to IGN, Buy D2R items PC director Joe Shely and general manager Rod Fergusson opened up on what Diablo 4 has in store for fans of the series, as well providing a glimpse into how the game's storytelling mechanics differ from previous Diablo games that came before it.