Spark Plug for 4AGE BT 20v engine is IK20.
For further information, u may refer this website
http://www.globaldenso.com/en/products/aftermarket/plug/basic_knowledge/heatrange/index.html
I've had my heart set on a twin turbo upgrade from my GTRS to 2835s. However, HKS recommended, in fact strongly suggested a T04Z setup instead. Problem is that at the time I was going for the 2835s, the T04Z kit sold...