The fourth quarter (Q4) typically starts on October 1 and ends on December 31 for most companies that follow the calendar year. This period marks the last quarter of the financial year and is important for financial reporting and analysis. However, some companies may have different fiscal years, causing their Q4 to start and end on different dates depending on their fiscal calendar. To summarize:

• Most companies' Q4 starts on October 1 and ends on December 31.
• Some companies have fiscal years that differ, so their Q4 dates may not align with the calendar year.

This is the standard for the majority of businesses following the calendar year.