ArcGIS Field Calculator (Python): Convert Domains from coded values to descriptions

domain_convert.txt

#Pre-Logic Script Code:
def checkValue(c, d):
    if c == 1:
        return "A"
    if c == 2:
        return "B"
    if c == 3:
        return "C"
    if c == 4:
        return "D"

#Field=
checkValue(!Field1!, !Field2!)