Soix, true that rear channels are not as demanding as front, but I think that is irrelevant here. The output of that 475 dropped significantly when put under a load, so any load, even with just 2 speakers, could cause the same issue.
I am retired now, but worked in the audio industry, with experience in numerous areas, one being the design and supervising the install of HT, and actually using Yamaha and others. I don't think the Yamaha's are any better or worse than others in this price range. No entry level receiver ever did well with floorstanding speakers, not enough amplifier. I can't seem to find something I read earlier, but the 4T was described as a not so dynamic speaker like some other floorstanders, and had dynamics more like a monitor. This would also make me question trying to drive them with the 479, especially using the 5C as the center. Sure, talk to Aperion, however keep in mind they are not going to say anything that would lessen the chances for you to purchase their speakers.
The Aperion speakers seem to get decent reviews, but I don't see the 479 being able to extract the performance from them, like trying to run a Tundra pickup with a 4 cylinder Corolla engine. I'm sure the combination would sound fine on elevator music, but for HT with dynamics it would fall short.
Again, if you want the system to sound like a dynamic HT, you'll need much better power for the Aperion's, or speakers like I have suggested for the 479.