Blue Buffalo (and most other brands of) puppy food is until one year. Royal Canin puppy food goes to 10 months. A lot of people free feed well into adulthood. There's nothing wrong with that. If Minnie is at a good weight & if you think she's a good judge of her food intake, then free feeding should be fine. Plus, she's tiny. You want to make sure that she has access to food when she needs it.
As far as the amount of food...it really depends on the dog. Check your bag, although a lot of times pups don't eat exactly how much is recommended on the bag. I would think that at 4 months & 2.2 pounds, she should probably be eating in between 1/4-1/2 cup a day.
I hope that helps
