This evening, Wizards of the Coast announced that the Mythic Championship IV Barcelona will be Modern Constructed and Modern Horizons Limited.
Mythic Championship IV Barcelona will take place on July 26-28, 2019 at Fira de Barcelona Gran Via in Barcelona, Spain. It will be the first Mythic Championship to feature a full season of Mythic Championship Qualifiers, the schedule for which was just recently announced, all of which will be Standard tournaments.
Modern Horizons is a straight-to-Modern set (meaning that it will not be Standard-legal) that will release on June 14, 2019. The set will be featured in three Sealed Grand Prix in June: GP Copenhagen, GP Washington DC, and GP Seattle.
The date, location, and format of Mythic Championship III, which will be the first MTG Arena Mythic Championship and precede Mythic Championship IV Barcelona, have yet to be announced.