There probably isn't much of a difference, if any.
My understanding is that as the T2 got bigger and taller (2 buckles, then 3, then beefier buckles) the T3 got beefier (stiffer plastic) and the T4 came along. Then the T4 went away, then the T3 disappeared from NA (but was still sold in Europe), and now the T4 is back. Why the T4 and not T3? Beats me.
The main differences between the T3 and T4 seem to be the presence of a powerstrap and stiffer plastic, or not. Hopefully the "new" T4 will be alot like the older T3, with a stiffish sole but flexible plastic in the ankle.