The cartridge heater is a replaceable heating element in the V4 Hot End. If your Hot End is not able to maintain temperature, it might be time to replace the heater. Because this component endures a large number of extreme heating and cooling cycles, it does have a limited lifespan. It is a good idea to keep a spare available. The "V4 Spare Parts Set" includes one extra cartridge heater.
V4 Cartridge Heater Specifications
If your Hot End is able to heat-up but your temperature readings appear inaccurate, it is most likely caused by a damaged thermistor (rather than a damaged cartridge heater).