Persistence of lochial discharge four weeks after delivery is not normal and you need to see a gynecologist for further evaluation. Possibilities are- the uterus
being larger than in a singleton pregnancy
takes longer time to involute in a twin pregnancy; some retained products of conception like placental bits; onset of menstruation
with erratic cycles or hormonal
imbalance. Please maintain a healthy diet, genital hygiene and get appropriate treatment. You may also need styptics, pelvic antibiotic and hormones. Hope this helps.