Hi lego, welcome to the forum.
True surround sound effects rely not only on your left/right axis, but also a front/back axis. Without surrounds, you'll be missing the front/back axis. So any surround sound effects are only going to move from left to right and you won't get any "depth" perception.
What that means is say there is a scene in which a helicopter is coming from behind and to the left of you passing overhead then to the front and right of you. Without surrounds, you are only going to get the left to right effect and not the behind to in front effect. That may not sound like much, but couple your sound perception with what is happening on the screen - you'll be seeing something the helicopter doing something that corresponds to how it is supposed to sound - for instance you won't see it in front of you yet, but you'll hear it as if it is coming from the front left. Then, as the sound travels to the center you'll see it come overhead on screen then pass to the right. So, what you are hearing won't completely match what you are seeing, and you'll be missing out on the true immersiveness that the surround sound effects are meant to convey.
Hope that makes sense.
As for the 4C v. 5C difference. The 5C is a larger center with larger woofers (tweeters being the same on both). That means the 5C can reproduce lower frequency sounds than the 4C (4C can get down to around 80Hz, the 5C will get down around 75Hz - both measured at +/- 3dB). That also means the 5C has a wider and deeper sound stage than the 4C. If you sit about 10 feet or more from the TV, we really start recommending the 5C over the 4C at those distances.
And in a 12.1 x 11.5 room, yes, I would think the 4T's would work out nicely in that room.
Mark
Aperion Audio Guru