You may want to re-measure, Mike !!!!!
First, here is a picture of the C3 and a C4 running side by side. I took this when I ran both of them next to each other on my 29 gallon tank before moving the C3 to my 38 gallon tank.
The C4 measures 8 and 5/16" along the back of the tank.
The C3 runs just a tad over 7" along the back of the tank.
The top back of the C4 is 2 and 1/2" above the rim of the tank.
The top back of the C3 is just under 2" above the rim of the thank.
I have an Aqueon brand 38 gallon tank that sits on a stand against a wall in the living room. The rim 3 & 3/4 from the wall. The glass is 3 & 7/8 inches from the wall. I tried to fit the C4 that is now on my 29 gallon on the 38 gallon but it would have made an ever so slight indentation in the wall if I tried to cram it in there so I ordered a second C3 rather than a C4.
In the end, I like having the twin C3's next to each other for "good looks." But really no big deal. Though the cost of the C3 and C4 were about equal due tot $7 extension tube needed on the C3, I just didn't want to tear down the tank to move it 1/4 inch.
The "factory" mechanical debris reducing and polishing pad are one unit. On the C3 it measures just under 5" x 5" or just under 25 square inches. On the C4, it measures just over 5 & 1/2" x 6 & 1/4 inches or just about 35 square inches.
I don't use the factory media. The polishing pad is more like 150 microns and just doesn't go "all the way" for "diamond studded" clarity of the water. Accordingly, I use cut to fit Acurel Debris reducing pad + AquaticLife 100 Micron Non-Woven Filter (polishing) pad for "canister style" results.
Many people say you can just use floss in an aqua clear and get crystal water with that "fish swimming in air" look. That is actually correct .... but there is clearly a step above that. When I had only the C3 & AquaClear 50 with floss running next to each other, the water lost its "edge" in a few days from when I removed the Aqua-Tech 30-60. As a result, I had to run the Aqua-Tech (temporarily on the front of the tank) ...... so the water was flowing about 600 gph
of flow in my tanks just to feed my obsession with having "conversation piece" sparkling clear aquarium water. Two C3's in tandem handle it well..... The AC
/50 is running on 1/3 flow just for BB & backup.