The T3/T4 turbo is a hybrid. It uses the turbine from a T3 turbo and the compressor housing from a T4 turbo. This allows the turbo to spool up quickly, and run more boost, which gives you more power than a typical T3 turbo setup without the typical turbo lag that you see with T4 turbos..
It's a great setup for anyone who is serious about power, but also likes to drive the car around town.
For power, most people run these at 12-20psi. With around 25psi at the track. If you want to go all out, these turbos are able to handle as much as 30psi of boost. Usually you'll be looking at gaining ~300whp with this setup after a little bit of tuning.