The usual active-participle pattern is فَاعِلٌ. When the root خ و ف enters this pattern, the medial weak consonant is replaced by hamzah in this environment, yielding خَائِفٌ.
خ و ف → فَاعِلٌ pattern → خَائِفٌ
خَائِفَةٌ is feminine; خَائِفُونَ and خَائِفَاتٌ are masculine and feminine plurals.
The hamzah is not a fourth original root consonant. It is a derived replacement for the medial wāw; the root is not خ ا ف.
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Why Khāʾif Contains Hamzah
Complete Conjugation of Khāfa–Yakhāfu: Hollow Wāw from the Samiʿa Class
The usual active-participle pattern is فَاعِلٌ. When the root خ و ف enters this pattern, the medial weak consonant is replaced by hamzah in this environment, yielding خَائِفٌ.
خ و ف → فَاعِلٌ pattern → خَائِفٌ
خَائِفَةٌ is feminine; خَائِفُونَ and خَائِفَاتٌ are masculine and feminine plurals.
The hamzah is not a fourth original root consonant. It is a derived replacement for the medial wāw; the root is not خ ا ف.