Seth absolutely stumbles backwards when Four approaches him, only to fall flat on his ass, as his attempt at processing all of this was interrupted.

Dear gods.
...He hated Four.
Sadistic son of a bitch.

It takes a moment but then...
Seth flips Four off.

His emotional resources were in the negatives and so anger and frustration moved to take the front of the stage, temporarily covering up his panic.
He'd still definitely have a meltdown later but for now...
He was just pissed.