The review I just saw only showed 3, but considering one of them I saw was the 13th Prime, it does expose a potential issue. The Prime Masters are on a rotating disc, where Amalgamous and Nexus are positioned on a completely different part of the disc to Prima. It looks like if you aren't mindful of that, that you can easily lose Prima.
While the 4 Prime Masters are a definite nice touch, I just can't get past that cardboard throne - not when you can be paying close to $200 for it.