Protein takes priority. If there is ample glycogen (stored glucose) and the body is getting the rest of its energy efficiently from fats, protein will always go first towards repair or building cells or enzymes. In that context, it hardly seems fair to assign it a “burn rate” of 4 calories per gram. It’s like saying the 2×4 studs that support the walls of your house can burn nicely if you run out of firewood. They will, but I prefer to burn other fuel first. At a minimum you need .5 grams of protein per pound of lean mass/per day on average to maintain your “structure”. If you are moderately active you need .7 or .8, and if you are an active athlete you need as much as 1 gram of protein per pound of lean mass. That’s at a minimum, but it’s on a daily average.