Today, the mailman sorted 57 letters during his first hour of work, 69 letters during his second hour, 81 letters during his third hour, and 105 letters

Today, the mailman sorted 57 letters during his first hour of work, 69 letters during his second hour, 81 letters during his third hour, and 105 letters during his fifth hour. Based on the trend, how many letters did the mailman sort during his fourth hour of work?