I would feed her once every 7-10 days until she is a year old. Then from 1-2 years old, I would feed every 2-3 weeks. Then from 3 years on, every 3 weeks to 1 month. It depends on the snake. Some eat more and do fine, some don't. Also males generally eat less and go longer without food where as females eat more and get fed sooner. Rosey my 6'+ girl just about tears her cage down if she has to wait over 3 weeks for food. Same with Vicky. If she isn't fed every 2-3 weeks, she goes bananas and will pretty much attack anything that moves lol.
As for size, a small rat for a 4 month old is way too big. I'm surprised she got it down and didn't puke it up. Rosey who is 3+ years old and 6'+ eats large rats and small rabbits. Vicky who is 4' and just over 2 years old eats small medium rats. At 4 months old, they were eating hopper mice and crawler rats( basically a fuzzy rat). Basically I didn't switch them over for a long time. I don't remember exactly when but you will know. When they have trouble constricting the food and can inhale it in about 1 min, and the food doesn't leave a small lump, then its time to upsize it.
Boas will always eat if they are healthy. Easiest way to know something is wrong is if your boa refuses a meal lol. Rosey hasn't refused a meal in over 3 years. Heck she has only refused 1 meal ever and Vicky has never refused a meal. She even sometimes tries to hit the front of the cage as I walk by with a rat for the BP and other boa. When they smell rats or rabbits, everyone goes on high alert and windshield wipes the front of the cages, whether they just finished a meal or are waiting for their turn. My poor beardie is in shock during this whole affair lol.