The forward roll is three notes - a bar or measure in 4/4 time is eight notes so three forward rolls do not fit. This roll pattern is eight notes long and therefore fits into a measure/bar of music. There are two main patterns one that utilises the second string and the other that utilises the third string. Both start with the thumb - 3, 1, 5, 3, 1, 5, 3, 1 from the second string the thumb starts with 2, 1, 5, 2, 1, 5, 2, 1. So basically we have two notes followed by two forward rolls = 8 notes. An alternative is that the two extra notes can be put at the end and then we would get 5, 2, 1, 5, 2, 1, 2, 1 with the thumb playing the last 2nd string.