Define Field D as a calculation field:
Case ( Field A = 1 ; Field C * 75 ;
Field A = 2 ; Field C * 65 ;
Field A = 3 ; Field C * 55 ;
Field A = 4 ; Field C * 45 )
( (4 - Field A) * 10 + 45 ) * Field C
Set up a related table of Field A values (1, 2, 3...) with a second field to hold the 75, 65, etc values. Then define a relationship linking the Field A values in this new table to Field A in your original table.
A calculation or auto-enter calculation can then be set up as:
Field C * RelatedTable::Factor
where "Factor" is the field in the related table that stores values of 75, 65, etc.
This last option allows a non filemaker developer to update what values are used by editing data in a table instead of having to redefine the calculation to use different values.