If you're into playing bluegrass, it's a good idea to learn about four finger mandolin scales.

As with any string instrument, learning to use all of your fingers is going to be an essential task. Although all four fingers aren’t always needed, there are times when using the 4th finger will make it easier to shift positions and to properly execute hammer-ons and pull-offs. Economy of motion is particularly important when key changes require large position shifts or when the other three fingers are fretting other notes.