Passing stools 4-5times can be normal at this stage.but presence of foul smell and mucus mighy suggest some mild gut irritation..which may be due to formula feed intolerance..or gut infection. I would suddest you to monitor baby for fever, feed refusal, irritability. Or lethargy. .. and also if the color of the stools turn green or frothy or bloody.
Next Steps
continue breast feeding.. and formula feeding if really nessasary .. ensure hygine while preparing formula feeds. consult a pediatrition if to add any medication.