OK, a little physics. Let's see what drives the code
Copper #6 has a resistance of .8 mohm / ft at 20C (that's round trip, both the black and red).
Loss is i^2 r, for 48A, 1.8 W / ft
For #4, it's 1.12 W / ft
The code writers said that it's safe to have 1.8 W when the wire is high-temp in a pipe...