Forward modeling reconstruction of multiple image system 4, highlighted in cyan in Fig. 1. The RGB images in the leftmost column are obtained by combining the HubbleF435W, F606W, and F814W filters. The next three columns, from left to right, show the model-predicted images obtained by using our forward modeling code GRAVITYFM (see Sect. 4) and adopting the deflection maps of our new reference model, the R21 model, and the CATS model, respectively. The green polygon in the top left image contains the pixels associated with the single image (4c) that is exploited by GRAVITYFM to determine the best-fit parameter values of the background source, while the other images (4a and 4b) are used as test images and predicted a posteriori. The cluster tangential critical lines, computed at the source redshift, are plotted in red. Magenta crosses give the positions of the cluster member galaxies included in each model.