I'm 18 & been having brown discharge after diarrhea for 4 days. Also have been gassy. Could this be IBS or signs of ovarian cancer?

Not likely. Certainly not compatible with ovarian ca. Possibly ibs but would need to review symptoms much more comprehensively. Ibs has to start somewhere, but 4 days of diarrhea by itself is inadequate for diagnosis. Much more likely to be normal variation, something you are eating, or mild intestinal infection. Brown d/c is a little more concerning. If it persists after diarrhea is gone get it checked.