If a source of power 4kW produces 1020 photons/second, the radiation belongs to a part of the spectrum called:

Power = n × Energy of Single Photon.
4 × 103 = 1020 × hν
ν = (4 × 103) / (1020 × 6.623 × 10-34)
ν = 6.03 × 1016 Hz
The obtained frequency lies in the band of X-rays.