The 4 Rakaats Sunnah before Zuhr is an emphasised Sunnah (Muakkadah) and one should make every effort to perform these 4 Rakaats before the Fardh. If one misses these 4 Rakaats, he should perform them after the Fardh Salaah.

A’isha (R) reported that when Nabi (Sallallahu Alaihi Wasallam) missed the 4 Rakaats before Zuhr, he used to read it after the Fardh.(Tirmizi)