Question:
Why is it always referred to as the 4 khalifs and not the 5 khalifs, since Imam Hasan’s 6 month khalifate falls within the 30 years mentioned by the Prophet SAW?
Answer:
Your premise is correct and was mentioned by Imam al-Suyuti in Tarikh al-Khulafa’. The answer to your question is because of the predominance (ghalaba) of the first four, and Allah knows best.
Hajj Gibril Haddad
