First off, we saw prototypes of this car before any even knew that a Yokomo SD car was in development, let alone existance. So I think it's rather ludicrous to claim that HPI ripped off Yokomo, or even Tamiya.
Yes, the car looks similar, but think about it for a second. How many ways can you do a shaft-drive car? I suppose you could flip it around and stick the motor up front, but I think that would unbalance the car too much. So really, just about every shaft car is going to look pretty similar. Stick a plate-type chassis on the TC3 (like the IRS one) and it doesn't look too different either.
If you look at the CAD drawings of the Pro 4, I can see right away that the suspension geometry is a bit of a departure from the Evo III and SD, both of which I have sitting in front of me right now. If you wonder why a lot of people have returned to the TC3 after trying some of the other shaft cars, look no further than the suspension. Most of these shaft-drives have such a similar drivetrain that you can almost remove it from the equation; it's the rest of the car that makes it work or not.