Better leave that Diablo 4 file directory alone. A new update from the series' global community development director Adam Fletcher has warned Diablo players of dire consequences should they attempt to modify their copies of the game, up to and including permanent bans.
In a post to the Blizzard forums, Fletcher wrote that "The Blizzard EULA explicitly prohibits cheating, bots, hacks, and any other unauthorized software which automates, modifies, or otherwise interferes with the game". That means "any game-modifying software" is "prohibited for use with Diablo IV," and woe betide anyone who tries to get around the rules: Fletcher says that "players who install this kind of software will put their accounts at risk for disciplinary action, which can include permanent suspension."
Fletcher actually called out a specific forbidden mod by name: TurboHUD4 (or Thud4, as I'm now going to call it). Curiously enough, Thud4 is a UI mod rather than some kind of tricksy collection of cheats and exploits, offering different overlays you can tinker with that do things like track your resources and missed items. It's not totally innocent, mind you: I suspect Blizzard isn't keen on its ability to reveal dungeon maps before you've explored them, but even that seems a relatively minor sin compared to some of the hacks that plague other online games.
The ban on mods like Thud4—and apparently all mods—might be a little counter-intuitive for Blizzard fans specifically. WoW players have been using UI mods to give themselves easier access to relevant information for ages (heck, we even have a guide to WoW addons on our site), for instance, and it's always been perfectly kosher.
Blizzard seems to have had a deliberately fuzzy enforcement policy in the past for its games, mostly turning a blind eye to mods that were technically not allowed but did little harm. Back in the Diablo 3 days, it wasn't uncommon to see streamers running around using a predecessor to Thud4, even though that too was technically forbidden at the time. There are still threads on the forums from back then made by people seeking clarity as to whether they were or weren't allowed to use it.
In light of that, maybe it's for the best that Blizzard is drawing a clearer line in the sand against mods like Thud4 for Diablo 4: It should hopefully mean less people run afoul of the rules out of ignorance. But I'm still concerned about how far-going that ban on "any game-modifying software" will be. Should players using reshade presets be worried? What about other overlay mods that do nothing but surface already-accessible info more clearly?
You'd think they'd be fine, but they are technically "game-modifying software". I've reached out to Blizzard to ask about this, and I'll update if I hear back. In the meantime, maybe stick with vanilla D4.