98 ford f150 wont start computer read out said number four piston is'nt fireing is it just a plug or more?

it tries to turn over but wont and it was hooked up to a computer and the read out said #4 piston isnt fireing so i thought one plug isnot gonna keep it from running just run a little bad so is itjust plugs a tune up or more of a problem im buying it from a guy and dnt wanna spend money on junk so please any info would help!!