Commentary
The words: يُؤْتُونَ الزَّكَاةَ (pay Zakah) in Verse 4 carry the injunction of Zakah, although the verse is Makki. This tells us that the original injunction of Zakah had already been revealed before Hijrah, right there in Makkah al-Mu` azzamah. As for the popular assumption that the injunction of Zakah was enforced in the second year of Hijrah, it refers to the arrangement of fixing its nisab (threshold), providing details of obligatory amount to be paid, collecting and disbursing it on authorized heads by the Islamic government. This was done in the second year of Hijrah.
Ibn Kathir while commenting on verse 20 of Surah Al-Muzzammil: وَأَقِيمُوا الصَّلَاةَ وَآتُوا الزَّكَاةَ (Establish Salah and pay Zakah - 73:20) has arrived at the same conclusion, because Surah Al-Muzzammil as one of the Makki surahs was revealed during the very initial period of the revelation of the Qur'an. This tells us that the way Salah and Zakah have been described mostly side by side in the verses of the Qur'an, its obligation had also taken effect side by side. Allah knows best.