What I am seeing this as is your 3.9~ volts is reading the voltage throughput on the battery minus the v-drop of the mod. The non firing tells me its an isolation problem.but when I read the voltage from post to post without a coil it reads the same as the voltage from terminal to terminal of the battery itself, and other attys won't work either, even ones whose 510s are much longer.
Keep in mind that an atty can be tested for resistance without being mounted on the mod...