Hey, good question! Honestly, kung 4 days nalang expire na ang inject mo, mas safe mag-wait nalang para sa peace of mind, a few of my mum friends actually said they prefer to get the next shot na bago pa maubos yung protection kundi hassle lang yung anxiety, di ba? Best talaga na i-ask mo agad sa OB mo kung okay na or if you need to use protection pa while waiting for your next dose, kasi bawat body and situation is different.