Calculate total in mysql
You can use IN(...).
Example:
SELECT * FROM table WHERE someValue IN (1, 2, 3, 4);
Tailored to your existing query:
SELECT `line`, SUM(`FormerBroken`), SUM(`FormerCrack`) FROM `line_check` WHERE `Month` = '2021-12' AND `Line` IN ('1', '2', '3', '4');