wah 4-1..? what i found out from the other celica owners never change the stock celica extractors. if u kept the original 1 change them back, take out the cat converter and the middle bullets, all the way straight flow to the S flow box next to the petrol tank then continue straight flow to the back. don't install a muffler also.. just an exhaust tip will do for a bit of design. i'm not too sure whats the diameter but i think it's 2.5" straight flow all the way to the s-flow then 2" till the back,