In a four-rakah fard prayer, the first two have required recitation, while the last two have a distinct ruling.
In four-rakah nafl, each of the four requires recitation; likewise, each of the three Witr rakahs requires recitation.
The contrast prevents mistakenly applying the last-two-fard-rakah ruling to nafl or Witr.
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Voluntary Prayers: Regular Sunnahs, Nawafil and Juristic Differences
In a four-rakah fard prayer, the first two have required recitation, while the last two have a distinct ruling.
In four-rakah nafl, each of the four requires recitation; likewise, each of the three Witr rakahs requires recitation.
The contrast prevents mistakenly applying the last-two-fard-rakah ruling to nafl or Witr.