@fmf saidYou're right that "is" follows the implied singular "sum", but in fact the whole sentence is invalid.
Four and four is eight. CORRECT.
The [sum of those two numbers] is X
My mother always insisted I was wrong about this.
Four and four are simply two nouns; try replacing them with any two other nouns. Cow and horse is/are livestock?
On the other hand, using symbols makes it a valid numerical statement. 4 + 4 = 8.