uh, hmmmm. A T3/T4 is a hybrid based off of Garret turbo's. There are probably 100 different combinations of a "T3/T4". But you have an IHI, not a Garret, so you don't have a T3/T4. Based on the GM, I'm guessing you have an IHI off of a GM 6.5 diesel. Really, unless the previous owner knows the specs, you'll probably need to take it to a turbo shop and have them pull it apart to see exactly what it is.