The last 8 Mile rap battle meaning?

Why did Eminem say
1, 2, 3 in to the 4
1 Pac , 2 Pac, 3 Pac, 4
4 Pac, 3 Pac, 2 Pac, 1
You're Pac, He's Pac, No Pac, None!

I watched the movie a many times and never understood why he points at
diffrent members of the free world and say that. Does anyone know????