Oo normal yan, mommy! Ang hormones mo na nag-relax ng ligaments plus the weight you're carrying is nagsstart na mag-strain sa lower back mo, happened to me din around that time. But since 4 weeks pa lang, it's still pretty early, kaya mas maganda talagang mag-check with your OB para ma-rule out kung may iba pang issue at ma-guide ka on safe stretches or positioning na makakatulong.