A 4 digit addends could have a 5 digit sum because of the rule of regrouping. Regrouping is a situation where carrying and borrowing is involves. So let’s have an example
8999 +5678 14677
so in this example, we added 8999 to 5678. 9+8 = 17, so we bring down 7 and carry 1 9 + 7 = 16 + 1 = 17 , so again we bring down 7 carry 1 9 + 6 = 15 + 1 = 16, bring down 6 carry 1 8+5 = 13 + 1 =14, since this is our last number, bring down 14.. => 14, 677
