exhaust pipe diameter....?

[proPIPING]

can anyone suggest to me the diameter of the exhaust pipe as i'm gonna make pipe for my project...the pipe will follow the ori evo123 route bcoz sometime i need to drive it to kl for tuning,the car setup is for drag..2.1l,gt turbo,custom g.box(kacuk),big blitz r33 intercooler,s.metal intake,standalone and bla3...in my head i wanted to use 4 inch diameter direct after the 3 inch flexible...any suggestion?

 

[140dB]

sideplant bypass valve str8 after the turbo

 

[proPIPING]

sideplant bypass valve str8 after the turbo

not very clear ar bro what u suggest