وَ«أَرْبَع» فِي قَوْلِكَ: «مَرَرْتُ بِنِسْوَةٍ أَرْبَعٍ» مُنْصَرِفٌ مَعَ أَنَّهُ صِفَةٌ وَوَزْنُ الْفِعْلِ؛ لِعَدَمِ الْأَصَالَةِ فِي الْوَصْفِيَّةِ.
Translation: In “I passed by four women,” «أَرْبَع» is fully declinable, even though it has descriptive use and a verb-like pattern, because its adjectival character is not original.
This exception matters: surface resemblance to an adjective or verb pattern is not sufficient to establish diptote status.