misfire #4 what will fix it on 2000 Mazda B4000

I replaced all plugs because it was running sluggish put on computer said #4 misfire how can I fix and what should it cost

Asked by for the 2000 Mazda B4000
Spark plug wires ➕ coil pack .. Possibly fuel injector... Can't give cost estimate! .. Contact local parts store/repair shop.