My 1993 mercury sable has been sitting an a garage for four years the battery was bad so I replaced.now when I go to start it it Wil clik one time but it won't turn over. Why won't it start?

Well its a possibility the motor is seized up. Or the battery connections are not good enough, Id clean up the connections as well as some of the grounds.

Sounds like the starter is bad. The one click would be the solenoid on the starter engaging but the starter does not turn. If it does not click the next time you try to start, then that would indicate the solenoid is sticking. Also ensure that the cable to starter is good and the connections are tight. The starter will have 2 wires. 1 heavy wire that runs the starter itself and a lighter wire that runs the solenoid. The second wire seems good. Also ensure there is not an inline fuse in the starter cable. Some cable incorporate the inline fuse in case the wire shorts out to the chassis.