We design a four InAs quantum dot system in which two leads are irradiated by terahertz light fields. The average current through the system is obtained by means of non-equilibrium Green's function theory. Typical photon-assisted tunneling is observed in the current energy spectrum. 100% polarization can be achieved by adjusting the intensity of Zeeman magnetic field or the amplitude of terahertz light field. As the frequency of terahertz light field is invariant, a zero current can be obtained by adjusting the amplitude of the terahertz light field, indicating an optically controlled quantum switch device. The spin polarization can be converted between 0 and 1 by tuning the energy level of quantum dots, suggesting a physical scheme of a polarization pulse device. Moreover, the system can be constructed as a terahertz detector due to the oscillation properties of spin polarization. The present work sheds lights onto the design of quantum computing and spin-dependent quantum devices.