t34 is a t04b [its back plate and comp housing size]
t4 cossy is a t04e bigger comp housing and back plate all fit same shaft and center bodys
theres 3 main comp housings sizes .50 ,.60,.70 [cossy t4]

to be fair you don't have to jump from a t34 to the t4 you can bore hosing and put bigger wheels in mine fave at the mo is a .48 turbine with stage 3 t3 shaft work's well
fast spool smaller turbine keeps torque good and bigg wheel passes a lot of air
plus i bore t34 for bigger comp wheel's again faster spool up

turbos only good on 2 liter on around 400+bhp

my turbo i'am using is a hx35 with a hybrid turbine which i machined my self for faster spool up these can run 3 bar of boost lol