We have characterized a heterodyne receiver based on an NbN hot electron bolometer integrated with spiral antenna as mixer and an optically pumped FIR ring laser at 4.3 THz as local oscillator (LO). We succeeded in measuring the receiver output power, responding to the hot/cold load, as a function of bias voltage at optimum LO power. From the resulted receiver noise temperature versus the bias voltage, we found a DSB receiver noise temperature of 3500 K at a bath temperature of 4 K, which is a minimum average value. This is the highest sensitivity reported so far at frequencies above 4 THz.