I should be a little surprised by this, but the original rumor that I saw discussed just such a 4-pack. Over time that morphed into "Oh, it will be 4 different Praetorian Guards", and now we have a reality check.
I'm a little surprised by the TFU Shadow Guard, too. That game came out almost 10 years ago, and while it was a big multi-media event, Lucasfilm seems to have moved on to other projects. And then this guard gets offered to us? Without Galen Marek/Starkiller being offered first in the 6" Black Series? I understand the tie-in with the Royal Guard and Praetorians, but I never got a strong sense of the Shadow Guard being tied into the larger saga in any way other than the TFU game.
As for the Senate Guard? I saw the image and it was tough to distinguish the finer details. It left me wondering if this is the TPM/AOTC version of that guard, or if it's the ROTS version of the helmet that's closer in design to the Emporer's Royal Guard.