Ay, totally understandable ang pag-aalala mo, lalo na first time mom ka pa! Sa amin, ang OB ko naman ay nag-ultrasound lang every trimester unless may concern, kaya nga some of my mum friends din ay ganyan ang experience, hindi every month, depende sa OB talaga at kung may specific reasons. But honestly, since you're already at 4 months at healthy naman ang previous scans, better to just follow your OB's judgment on this one kasi siya na ang nakakita ng baby mo sa ultrasound; pero if it's really making you anxious, don't hesitate to ask directly kung when ang next ultrasound or if may specific signs to watch for in between, most OBs appreciate when you communicate your concerns na, and they can explain better kung bakit ganyan ang approach nila.