As stated on the IRS Form 8915-E for line 4 -
Add lines 1 through 3 in columns (a) and (b). Complete column (c)
if line 4, column (b), is more than $100,000. Otherwise, leave column (c)
blank
The amount of $100,000 is already entered on Line 4 of the Form 8915-E.
Go to this IRS website for Form 8915-E - https://www.irs.gov/pub/irs-pdf/f8915e.pdf
