We discuss the security of QKD protocol with four-state system based on single spatial and frequency non-degenerate down converted photons. Simple schemes for biphoton generation and their deterministic measurements are analyzed. Three main incoherent attacks (intercept-resend, intermediate basis and optimal attack) on QKD protocol in Hilbert space with dimension D = 4 using three mutually unbiased bases were analyzed. It has been shown that QKD protocol with four-dimensional states belonging to three mutually unbiased bases provides better security against the noise and eavesdropping than protocols exploiting two bases with qubits and ququarts.