We consider generalized Paley graphs Γ(k, q), generalized Paley sum graphs Γ + (k, q), and their corresponding complements Γ(k, q) and Γ+ (k, q), for k = 3, 4. Denote by Γ = Γ * (k, q) either Γ(k, q) or Γ + (k, q). We compute the spectra of Γ(3, q) and Γ(4, q) and from them we obtain the spectra of Γ + (3, q) and Γ + (4, q) also. Then we show that, in the non-semiprimitive case, the spectrum of Γ(3, p 3ℓ ) and Γ(4, p 4ℓ ) with p prime can be recursively obtained, under certain arithmetic conditions, from the spectrum of the graphs Γ(3, p) and Γ(4, p) for any ℓ ∈ N, respectively. Using the spectra of these graphs we give necessary and sufficient conditions on the spectrum of Γ * (k, q) such that Γ * (k, q) and Γ * (k, q) are equienergetic for k = 3, 4. In a previous work we have classified all bipartite regular graphs Γ bip and all strongly regular graphs Γsrg which are complementary equienergetic, i.e. {Γ bip , Γbip } and {Γsrg, Γsrg} are equienergetic pairs of graphs. Here we construct infinite pairs of equienergetic non-isospectral regular graphs {Γ, Γ} which are neither bipartite nor strongly regular.