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