Best-fit BLR photocentres of our four targets. The columns from left to right show the photocentres from the data, the “2-pole” and “null” photocentre models (described in Sect. 4), and the photocentres from the best-fit BLR model of each target. The red cross refers to the continuum photocentre. The direction of the radio jet is shown as a green solid line. The black arrow shows the position of the best-fit “null” model fitted from the best-fit BLR model. The ellipses around each centroid refer to 68% (1σ) confidence intervals. The colours refer to the corresponding velocity of each spectral channel. The (significant) separation of the red- and blueshifted photocentres confirm that we have resolved the BLRs of our targets. For Mrk 1239 and IC 4329A, the perpendicular alignment between the radio jet and red-blue photo centres from the “2-pole” model fitting of the data and the straight alignments of their model photo centres indicate that their BLR is rotation-dominated. On the other hand, for Mrk 509 and PDS 456, the radio jet and red-blue photocentres are not closely perpendicular, and the model photocentres are curved, which suggests that their BLRs are radial motion-dominated.