The "Mergedverse World" Event begins right before Fiercy Forces,
where the created Avatar - Silent the Feral Lynx, is on the mission to take with her
the Night Guards...
She's like Shadow Sonic "Midnight", which he's taking the Golden Sonics with him.

Dr. Golden's "Preparation" Evil Plan... has begun.

Luigi is absend in FNaS4 "Mergedverse World" because in that time, he is trapped in the Maniac Mania Arcade Machine, which is located in Dr. Golden's "Preparation" Castle.

The Event happens at the moment where Luigi escaped from the ruined...
nightmarish Pizzeria and lost his cap.

Exactly... The End of FNaS4.

But this time... Nightmare Clones expect for the protagonist Silent to arrive.
As it was all planned...

Alright, that's the end of spoilers.