Diablo 4 does not allow inventory expanding but Blizzard is working on a change - LemmyWorld
Blizzard admits to being aware of requests from the Diablo IV community
regarding inventory expansion, though it doesn’t look like the feature is coming
anytime soon.
I always liked how they solved this annoyance in Torchlight, where you could load up your pet with loot and order it back to town to sell/salvage everything.