Would like to know why it won't pull. on 2001 Mazda Tribute

It happened once before,,approx 2 months ago. this is the 2nd time. The diagnostic ( don't know the code ) but it says it's in the 4th cylinder. Thanks so much for any help!!

Asked by for the 2001 Mazda Tribute
what is the code number? poss po304, which would be misfire #4 cyl
What would make it misfire?,,,And would misfiring make it not pull? I had a new spark plugs put in when it happened the 1st time,,,thanks,,,I appreciate it!