Photonic crystal is a new artificial material. Its unique optical properties enable it to be used in fabricating novel optical devices. In this work, a 4-port coupler is proposed theoretically in silicon based on two-dimensional silicon photonic crystals. The optical properties of the 4-port coupler device have been demonstrated for TM polarization by finite-difference time-domain method. Transmittivity in each output port of the device is given as a function of coupling length. Results show that propagation direction of the input light beam can be controlled by changing coupling length of the device. Further studies indicate that the output optical power in each output port of the device can be tuned by reducing the radii of the silicon rods in the photonic crystals. The properties make the device find its potential applications in all-optical switching systems with wavelength-selective and tunable properties.