Which scenario does NOT qualify under the definition of "substantial understatement" for tax purposes?

The scenario involving an understatement of $4,000 does not qualify as a "substantial understatement" for tax purposes because it does not meet the threshold set by the IRS. A substantial understatement is typically defined as an understatement of income tax that exceeds the greater of 10% of the tax required to be shown on a return or $5,000 for individuals.

In this case, if the understatement is only $4,000, it falls short of both the dollar amount threshold and the percentage threshold, thus it cannot be classified as substantial. Meanwhile, the other scenarios involve understatements that either exceed the dollar threshold or meet the conditions that qualify as substantial under IRS regulations. This understanding is crucial for tax reporting and compliance, as it helps individuals and businesses avoid heavy penalties associated with substantial understatements.