Watch the Alternating Thumb Roll online guitar lesson by Ben Wright from Bluegrass Banjo for Guitar Players
The alternating thumb roll is similar to the 4 note roll but alternates what string the thumb is hitting every time you start it. This ends up making your first 8 note roll, which is standard for most banjo playing in 4/4 time. Make sure you keep all the notes evenly apart and try not to put too much of a bounce into it. Once you eventually increase the tempo you're playing, a bounce won't translate very well.