The 'carrate' tag has been discussed a few times here, I believe I did the last Wiki update on the page "Index of Tags & Containers" for that tag.
I tried a test on this asset;
<kuid2:647907:102362:1> YARNish L2 DL1 -Si +Sh, TH SM - CDE
I cloned it and set the 'carrate' to 1. What I found is that the it worked per the math stated, and ended up with more traffic than using 'carrate' 4. So the statement 1,2 and 3 don't work is false. I then tried to find where, if at all, it was used in the factory .gs files to get more info, and I could not find it there, so it must be controlled at an upper level.
If using 1,2 or 3, I would be worried that someday down the road it all of a sudden quits working, as in reserved for future use.