Hi,
Bruises on the breast can often happen from an injury that we sometimes don't even remember how it got there.
If you discovered this bruise just 4 days ago, then it should get better with time.Try not to feel it or rub it because it could get worse.Try cold compresses,but if this bruise gets worse or does not improve in a couple of weeks,then it's best to have your doctor evaluate it.
Best wishes...