I believe you have to have updated suspension some of which is available as a factory option or aftermarket to go above 3900kg, up plating obviously required.
If you are going to run 4000kg GVM then I wouldn’t be too concerned about engine performance. I run close to 3500kg all the time and it’s refined and fast (for a van.) You do always have the option of a stage 1 remap too if you are concerned.