Ah mama, lower back pain at stiffness during pregnancy ay common talaga, lalo na habang lumalaki ang baby, pero since it's causing you significant discomfort at you're only 4 months in, definitely check with your OB first para ma-rule out kung may ibang concerns at matanggap mo ang proper advice for your stage. A few of us mums swear by prenatal massage or pelvic support, pero talk to your doctor muna before trying anything, ha? Take care of yourself!