I think you misunderstand the idea of the 58 post. It's not that each player can post 4 times, it's that each player can post once per day.
Day 1 has 13 players, so 13 posts.
Day 2 also has 13 players, so 13 posts.
Day 3 has 11 players, so 11 posts.
Day 4 has 9 players, so 9 posts.
Day 5 has 7 players, so 7 posts.
Day 6 has 5 players, so 5 posts.
