How much food should my 4-month-old Husky Mastiff puppy eat?

As a half-Mastiff, Ruby is likely going to be a pretty large dog, so I do recommend feeding her a large breed puppy food for the first year of life. Every food is formulated differently with respect to the density of calories in kibble. For this reason, follow the chart on the bag of food to match Ruby's age and weight, as this will provide the amount of food that she should have in a given day. You can then divide that amount into 3 meals as you are currently doing. I typically recommend feeding Science Diet, Purina ProPlan or Royal Canin diets, as these are high quality foods formulated by veterinary nutritionists and backed with clinical trials. If you choose to change Ruby's diet (now or at any point in her life), be sure to do so gradually by mixing her old and new foods together over 5-7 days. I hope this helps and that all goes well for Ruby!