This trick involves starting with a stack of four coins held edgewise
the thumb and the first three fingers and finishing with one coin held
between each pair of fingers.
Before learning the Roll Down I strongly recommend becoming proficient at the Edge Walk with a two coin stack. If you are comfortable with that trick then learning the Roll Down will be a lot easier.
Once you have a solid Edge Walk you should start working on the Two Coin Splits and Three Coin Splits. These are breakdowns of the full Four Coin Roll Down and will teach you all the basic moves you need for the full trick.