Perovskite-type ferroelectric Pb(Zr1−xTix)O3 (x = 0.3; 0.4) compounds were investigated at high pressure by means of time-of-flight neutron powder and angle dispersive x-ray diffraction (NPD and XRD) with special emphasis on the interactions between the ferroelectric Γ4− and antiferrodistortive R4+ instabilities. A simple and fast way for probing at the long-range level the ferroelectric Γ4− instability in perovskite compounds is reported, which unlike Rietveld refinement neither requires numerous reflections to be collected nor a structural model to be chosen. This method, built upon the fact that the imaginary component of the structure factor Fhkl of general hkl reflections cancels itself in centrosymmetric perovskite structures, consists in the monitoring of the integrated intensities of these reflections directly on neutron powder diffraction profiles. Thanks to this powerful probe, the passage by a metrically cubic unit cell at high pressure, which appears to be a common feature of PZT ceramics, is shown unambiguously to originate from the competition between polar cation displacements and antiphase tilting of (Zr,Ti)O6 octahedra. Furthermore, based on the high-pressure behavior of the lead isotropic displacement parameter, significant static disorder is found to persist a few GPa beyond the critical pressure of the ferroelectric-paraelectric phase transition, corroborating recent findings reported for the two other cations (Zr, Ti), namely that this phase transition has an order-disorder component.