Complex sentence: Ni’mat Allah differentiates between four main groups of Pashtuns--descendants of the three sons of the putative ancestor of all Pashtuns, Qays Abdul Al-Rashid Pathan, together with a fourth group.
Split into simpler sentences: Ni’mat Allah differentiates between four main groups of Pashtuns. These are the descendants of the three sons of the putative ancestor of al Pashtuns, Qays Abdul Al-Rashid Pathan, plus another, fourth group.