I changed from a 3 wire dryer cord to a 4 wire dryer cord.?

Check your connections. It sounds like you are only running on one hot leg, meaning you are running a 240V dryer at 120V. Check the wiring at the dryer, and if that is good check the connections at the receptacle (with the breaker off). Also make sure that you removed the ground jumper from the center (white) terminal, and connect the ground wire from the cord to a screw on the chis. Many use the screw for the cover as a grounding terminal. Hope this helps.