4,821 + ? = 10,920. How do you find the missing part? This 3rd grade 4-digit addition sentences worksheet starts with a worked example, then gives eight column problems. In some, the sum is missing and your child simply adds. In others, a missing addend hides in the problem, and the trick is to subtract. Numbers go up to four digits, and a few sums reach five. A second page gives the answer key.