Since we do not know what the expected shape of this distribution is going to be, a nonparametric method using ranks is in order, and there is some reason to think that frequency rank regardless of n-gram size will be useful.
ha, Sicilia-Garcia, Ming and Smith (2002) show that Zipf's law can be extended to the combined frequency distribution of n-grams of varying length up to rank 6, which entails that the relative rank of words in such a combined distribution provide a useful estimate of relative probability.