Answer
The sum of the 2 photon frequencies equals the frequency of the single photon.
Work Step by Step
Energy is conserved. The sum of the two photon energies equals the single photon's energy.
$$E_{1} + E_{2} = E_{3}$$
For photons, E = hf, where E is the energy, h is Planck's constant, and f is the photon's vibrational frequency.
$$E_{1} + E_{2} = E_{3}$$
$$h(f_{1} + f_{2}) = h f_{3}$$
$$f_{1} + f_{2} = f_{3} $$
So the sum of the two photon frequencies, emitted when the electron transitions from 4 to 3 and then from 3 to 1, is equal to the frequency of light emitted when the electron transitions from quantum level 4 straight down to quantum level 1.