I'm using field calculator in ArcGIS Desktop 10.2.1.

I have field1 which has a values of "A_B" (example).  I created two fields out of this, values to the left of the underscore go in fielda and to the right of the underscore go in fieldb.  I used the code below in field calculator to separate them out into two fields.

fielda =

    field1.split("_")[-1]

fieldb =

    `field1.split("_")[-1]`

I now have field1 with values of A_B_C_D.  I need to separate the A_B_C_D into four separate fields using field calculator.  fielda = A, fieldb = B, fieldc = C and so on.
I'm not sure how to go about this.  I basically need to parse the values by the underscore.