Article 140 establishes the statutory deadline for filing the annual income tax return. Taxpayers must submit their return within four months from either the end of the tax year or the end of the relevant accounting period (if multiple periods exist, the earliest end date is the benchmark). This strict four-month window ensures that the Tax Authority receives financial data in a timely manner, allowing for prompt assessment and revenue collection. Failure to meet this deadline can trigger administrative penalties, making it a critical date for Omani tax compliance management.