Why is the Inductive reactance formula (XL=2πfL) not working for me?
Your inductance has an impedance of j50 ohms at 3620 Hz.  Your generator has a 50 ohm resistance at that frequency.  Using the standard voltage divider equation, and taking into account the imaginary value of inductor imepedance yields a voltage ratio of 0.707.  That value multiplied by your 4 volts yields an output of 2.83 volts which is close to your measured value of 3 volts.  The important point is that you must take into account the phase angle of any impedances that you use as a voltage divider.  In this case, your generator impedance has a 0 degree phase angle while your inductance has a 90 degree phase angle.