The T4 will NOT fit in a T34 rear housing, the cores are all the same on T3, T34 and T4's but as stated above the T4 has a larger turbine wheel so wont fit into the smaller T34 housing, front ends are all different even down to 3 sizes of T3 (escort turbo, 2wd cos and 4x4 cos) T34 is larger and T4 is larger again even though all the cores are the same,