In this fourth paper in a series on stochastic electrodynamics (SED), the harmonic oscillator-zero-point field system in the presence of an arbitrary applied classical radiation field is studied further. The exact closed-form expressions are found for the time-dependent probability that the oscillator is in the nth eigenstate of the unperturbed SED Hamiltonian H 0 , the same H 0 as that of ordinary quantum mechanics. It is shown that an eigenvalue of H 0 is the average energy that the oscillator (...) would have if its wave function could be just the corresponding eigenstate. The level shift for each unperturbed eigenvalue is found and shown to be unobservable for a different reason than in the corresponding QED treatment. Perturbation theory is applied to the SED Schrödinger equation to derive first-order transition rates for spontaneous emission and resonance absorption. The results agree with those of quantum electrodynamics, but the mathematics is strikingly different. It is shown that SED demands discarding the ideas of quantized energies, photons, and completeness of the Schrödinger equation, Finally, an intuitive physical SED model is suggested for the photoeffect and for Clauser's (2) coincidence experiment. (shrink)