The battery is mostly charged
.5 ohm parallel coil build
and the brand of battery is an Imren 18650 3.7v 2500mah 40a
Not sure if you know but, on a mech mod the charge of the battery is everything..Its not like the regulated types at all...you start at 4.2v then its all down hill from there..Always make sure the battery is FULLY charged before using it.
I see one of 2 things possibly wrong.
1.) The battery isnt charged enough as you might think it is.
2.) A possible bad connection somewhere not supplying you the full voltage.
The first process of elimination is easy...fully charge the battery.
The second one i really cant help with that much as i dont have the device in my hand to check for a connection issue.
Im not familiar with the four nine but some have adjustments for the battery connection on the bottom firing button and the 510 pin on the top to connect to your atty...both of those have to be adjusted proper for full voltage ,if this is your case.
Thats about the best i can help you with.