It's like the trick with the display-pixels, RED is counting "sub-pixels", others do not, therefore RED has higher resolution, just like a 920000pixel-LCD (640x480xRGB) has a higher resolution than a 307000pixel-LCD (640RGB-pixels x 480RGB-pixels)...
Well, not really. If it has three different sensors, offset to give a combined 4k image, then that is really almost exactly like what regular still digital cameras do. Whether other cameras output full RGB or interpolated RGB is irrelevant here. The important part, for that spec, is what image is output, not how they got there. One can argue that it has less resolution than other cameras or not, that is a separate matter.
In other words, unless you consider a 5D Mark II to have 7MP, the RED has 4k, since it does the same interpolation. In fact, it is even better, since the three sensors means that the three sub-channels actually sample larger areas, and are therefore more representative.
However, if the three colour sensors are *not* offset, but deliver exactly the same view with 1920x1080 resolution, then yeah, they are hyping their tech beyond believability. I presume that they are offset, for exactly this reason.
In fact, the same measuring standard is exactly why Foveon's 14MP claim is garbage: the sensor outputs 4.7MP, with full colour information, not 14MP. (Upressing an entire image is something completely different than interpolation between offset colour signals.)