just because its a t4 doesnt mean its a direct replacement... the standard on3 turbo i believe is a 2.5in out and 3in inlet or something..t4s range from 2in inlet and outlet to 6in inlet and 4in outlet..the on3 standard turbo is actually a really small 62mm or whatever it is.. i would do a t4 billet 67mm with a 4in inlet 3in outlet..make better power and better spool on same boost since its sucking in more and air putting more air out..
so if you get a t4 will it bolt on to the flange yes..will it bolt on to the vband down pipe,filter fit and coupler fit on the out let..if you get the exact spec'd size turbo it will.. but like i said i wouldnt..
if you get a slightly bigger turbo with bigger inlet and outlet all you have to do is get a filter with a bigger inlet and a another coupler for the outlet..so not to challenging at lol..