gggggggg:
How low can the source arduino power get before it will no longer function?
I have no idea. Some folks claim just north# of 6V is required on Vin but your testing contradicts that.
So in theory, if my VCC drops to 4.8v,...
In theory, that would be impossible. It's a 5V low dropout regulator so the cutoff should be at 5V.
Is there a real way to test this?
The first course of action is to get an opinion from someone who understands the switching circuit and voltage regulator (someone who isn't me). If you don't get a reply in a reasonable amount of time, I suggest posing your question in the General Electronics section (or spend some time searching the forum; I believe the topic has been discussed).
# For those living in the Southern Hemisphere that would be "just above".