I don't pay much attention to the protein and serving size numbers, such as 4 grams of protein for 1/4 cup (or 30 grams), on the flour bags because of rounding factors that make it impossible to know the exact protein percent. Instead, I look for more specific protein numbers from the miller, which is usually specified as x grams protein per 100 grams of flour. So, even though several flour bags can all say 4 grams of protein for 1/4 cup, the actual protein contents can vary because of the rounding factors. I often see flours from different millers with identical Nutrition Facts, yet I know the flours are not identical.
I don't know what percent of the total protein content of a given flour ends up as gluten when the flour is mixed with water and agitated (e.g., mixed or kneaded). Gluten is made up of two protein components of flour--glutenin and gliadin. However, there are other protein components in flour that have little or nothing to do with gluten. I would imagine that gluten represents the bulk of the protein content of a flour, I just don't know the percent. And it is possible, and maybe even quite likely, that the percents will vary from one brand of flour to another, even within the same category (e.g., bread flour, all-purpose flour, etc.).