I have been on metronidazole for 4 days now for an infection in my gum, but the swelling in the gum hasn't decreased. does it need drained?

Gum infection : It depends on the cause of the infection. There is no way to tell without a visual examination, x-rays and additional information. Make sure your return to your dentist or periodontist for a post-op exam to have the infection re-evaluated after you are finished with the meds.

Gum infection: Should have begun to reduce symptoms by now. If you have infection requiring drainage it's probably from infected tooth. Depending on cause of infection you may need to continue until all prescribed meds are taken, take a second course of metronidazole, be switched to a different antibiotic, have gum surgery, have abscessed tooth treated, be referred to specialist, etc.

Metronidazole: Metronidazole is not usually prescribed for dental infections, since it affects bacteria usually associated with infections of vagina, stomach or intestines, liver, skin, joints, brain, heart, and respiratory tract. Is this an antibiotic that your dentist prescribed, or one that's left over in your medicine cabinet? You do understand that antibiotics do not resolve dental problems. Visit dentist.