collapsing solenoid magnetic field limiter
15 volts across 4 ohms is a solenoid current of 3.75 amps. When the contact that controls the solenoid opens an arc will form with a current of 3.75 amps. If your contact is rated at 100 volts when switching coils you need a snubber that can handle the current of 3.75 amps without producing more than 100 volts so this is a resistor of 26.6 ohms. You can connect it across the coil via a normally reverse biased diode that has a peak rating of at least 5 amps and you should be OK.