Hay, mama, please go see your OB as soon as you can, pain sa taas ng tiyan combined with back pain while pregnant needs to be checked properly, lalo na 4 months ka na. Hindi ko kaya sabihin what's causing it, but your doctor needs to make sure you and baby are okay. A few of my mum friends had different reasons para sa ganitong pain, iba ligament stretching, iba something else entirely, kaya talaga important na tama-check lang agad. Wag mag-atubili, mama, this is not something to wait out. Let me know how it goes, ha?