LMR400 outer conductor should be a bit less than 10 milliohms per foot
at 4MHz.

Rule of thumb: for copper, assuming thickness at least a couple of
skin depths, RF resistance in ohms per thousand feet is about
sqrt(freq(MHz))/(diameter(inches)). Aluminum will run 25% more or so.
I've assumed the aluminum foil shield of the LMR400 is at least 3 mils
thick, and I've neglected the overbraid.

Cheers,
Tom