My car has a five prong and my trailer has a 4 prong, how do I connect?

I am trying to get my trailer lights working. My car has a 5 way flat and the trailer has a four way flat connector. When I plug them in the lights do not come in on the side but when I turn the blinker on they start flashing on both sides. The one cable that does not match up is the blue one on my car. The trailer does not have ablue cable as it has a hydraulic brake actuator that is not attached to the wiring system. Extremely confused on what to do, please help!