4x CREE XP-E2 M2/M3 on 23mm star PCB
You can't light 4 x white LEDs in series with a 3.7 V source.



Figure 1. The Cree XP-E2 datasheet shows the relationship between voltage and current.

The graph shows that for one LED you would put > 1000 mA through the LED and eventually this will destroy it - probably fairly quickly.

With four LEDs in series on your 3.7 V supply you have &lt; 1 V per LED so the current will be only a couple of mA and the LEDs may be just visible in pitch darkness.



Figure 2. All LEDs have a non-linear relationship between current and voltage. For small LEDs the forward voltage, Vf will be quoted at some nominal current - typically 10 or 20 mA. Source: LEDnique IV curves.

Note that Vf varies with the colour of the light emitted because the wavelength is a function of the dopant added to the semiconductor.