Not sure why you are only getting 3.7v at full charge, but it should be 4.2v - or, as Gully said, just under, at 4.1v. Are you sure it is fully charged?
Over its use, your voltage reading will decrease as the battery 'drains' ...and, more importantly, how you use it will dictate how fast that decrease happens. The battery is rated at 3.7v so any voltage setting above that will drain your battery faster than it is rated (approx 1 hr for every 100mAh at 3.7v) ...the higher above you go, the faster the drain. Many vapers tend to prefer lower ohms
coils (1.8-2.4v) - which are typically best used at 3.7-4.5v - over higher ohms
coils (2.8v and above) - which are typically best used above 4.8v - on a single 18350 battery setup; otherwise, it's not going to last long between charges. People who use higher ohm coils above 2.8v are almost always using a 18650 or a stacked-18350 setup.
Although the actual bottom limit on the 18350 is 2.5v, I believe the Provari - I'm not 100% certain of the exact numbers - will start blinking its button light to indicate 'low battery' when the battery gets to around 3.4v (?) and will shut off completely at around 3.2v (?).