High speed parameters for transistor
If I would get a transistor to switch at 400 kHz, then I would look for a transistor with a bandwidth of at least 400 kHz, I'd probably look for a transistor with 1 MHz for some margin because the real world is messier than in theory.

But what I'd also look for would be the minimum \$h_{fe}\$, because that will greatly affect the efficiency of the BJT, or what kind of things you can use the BJT for. Also the maximum amperage (through Ic). But since your question is screaming of much effort put into the question, I don't know what other kind of parameters you should be looking for, rather than 400 kHz bandwidth. 

If you are going to make something that needs a lot of amp's, say more than 0.5 A, then I'd strongly advice you to use a MOSFET instead of a BJT.