I've been working on this with someone else, and here's what we've figured out....
Electronically, they are the same. Same FCC certifications, all the part numbers I've looked at are the same, at least when comparing similar wattage models. The TK-x400 models appear to be the same electronically as the TK-x400 protalk models (2 watts). The TK-x402 models appear to be identical to the TK-x400 models with the only difference being in the RF board having to do with the 2 watt vs. 5 watt ouput.
The firmware is the same between the TK-x400 and the TK-x402 radios.
The original product release info for the x402 protalk radios mentions that they can be programmed using KPG-160DK.
So, the guy I was working with on this tried pusing the non-ProTalk firmware into a ProTalk radio and it took it, however it still says it's a "protalk" radio. So there is something. I haven't heard any more about this, and I don't have one of these radios myself or the time to work on it.
Seems like if all you want to do is take a ProTalk radio and program your own custom frequencies into them, it can be done using the KPG-160 software.