Shounen ga Otona ni Natta Natsu Episode 4

A shocking truth!? The protagonist, Kirishima Ryuki, has noticed something. He begins to wonder why Reiko has never shown him her room for the past four years.
The answer is here… When he finally decides to open the door, he finds it there! Reiko’s true nature is revealed, and the more she strays, the more she is reminded of her past, and in the midst of the chaos, he feels a sense of liberation…
His feelings grow so strong that he can no longer turn back. Their secret will continue even after they become adults.