Diablo 4 is being built with expansions in mind and will have in-game purchases for cosmetic items only, according to Joe Shelly, Lead Designer over at Diablo 4. Joe handles "combat systems, encounters and dungeons" in Diablo 4.
A few other interesting points made by Joe are that max level in Diablo 4 will be 40 at the moment, but this could still change as development continues. He also mentioned that keybinds can be customised in Diablo IV, for those advanced Diablo players that need to setup their key shortcuts to enable them to play to their strengths. So you can move skills to where you need them on the keyboard.
It also seems that the skill trees in Diablo 4 are relatively limited, with a view to the characters customisation and progression coming more from the legendary equipment they find during the game. As these powerful items, when chosen to be equipped by a player, will effect the play style and abilities for that character.
And then Allen Adham was reported to say that they are working towards making Diablo IV's multiplayer capable of cross-platform play. So you can join buddies for a session even if they are playing on PC, when you are on XBox or PS4. This hasnt been completely implemented yet, as he suggested that they are working through issues, but cross platform is the intent for D4.
We also mentioned recently that Diablo 4 will be always online. Not sure if you remember the mess that caused when Diablo 3 released and Red Dead 2 recently, but there we go.
We have also spotted that there is a new place for us all to signup to for Diablo 4 news and special offers, over on the diablo 4 website. We can only assume that this will also be the same list of people's emails that will be included/considered for any future Diablo 4 beta, when/if they eventually run a beta.
So head over and sign up if you want all the latest information on Diablo 4 as it comes out from Blizzard and maybe get included into a Diablo 4 Beta.