One UI 4 will probaly hit the current Flagships in November or December 2021. Beta testing hasn't started yet. Expect stable general availability to supported models next year.
According to Member ice universe in twitter. Samsung's internal One UI 4.0 beta will be pushed to Samsung employees. He/She adressed it, by august 27. 2021 and the Galaxy S21 series will be the first One who will gonna get the ("Beta Version Of One UI 4.0") and yeah as what @jamesrred said the One UI 4 will gonna hit the current flasgships Galaxy phones.
