input
function returns a string.
You can convert this string to int and then compare it to 4, 6, or 12:
x = int(input("What die do you want to roll? 4, 6 or 12?"))
if x == 4:
// whatever
elif x == 6:
// whatever
Or you can compare x
directly to a string:
x = input("... whatever ...")
if x == "4":
// whatever
elif x == "6":
// whatever
And one other thing: there is no need to end lines with the semicolon in python. Makes Guido van Rossum happy.