Add fields based on Dropdown Selection

I am trying to find out how to add fields based on a related drop down selection?

I have (badly) attempted to show below:

CALCULATED FIELD 1 ( The addition of corresponding fields for selection1)

If field[0] = "selection1" then add field2[0]
If field[1] = "selection1" then add field2[1]
If field[2] = "selection1" then add field2[2]
If field[3] = "selection1" then add field2[3]

CALCULATED FIELD 2 ( The addition of corresponding fields for selection2)

If field[0] = "selection2" then add field2[0]
If field[1] = "selection2" then add field2[1]
If field[2] = "selection2" then add field2[2]
If field[3] = "selection2" then add field2[3]