Well you will be able to play the beta as early as this Friday, June 21st if you are a PlayStation Plus member. The new expansion will be coming in three months, and Capcom wants to make sure the servers are good and the bugs are out of the way before September 6th. The best thing about the beta is you will not be required to own the base game to participate.
Capcom had Iceborne playable at E3, and the beta will contain the same content. You will be able to do three quests and have access to high-level items for the fights. The beta will be available at 3 am PDT/6 am EST from June 21st-24th for PlayStation Plus members. Everyone else on PS4 will be able to play from June 28th to July 1st. No word on Xbox One or PC betas in store.