As much as I love grilling and live fire cooking, these aren't the most reliable ways to cook something this big. Ovens (or sous vide) are much friendlier in this respect. Sear it off on all sides and then throw it in a low oven, like 225F, and monitor the temp with a probe. Pull it when the core hits like 123F or so, crank the oven as high as it will go with convection and let it get good and ripping. The rost will carryover while you're waiting for the oven to thoroughly soak in therms. Then throw it in and let 'er rip until the crust is looking good.
You can do something similar with sous vide, but I like to go for a higher bath temperature than I would for a standalone rib steak. People like their roasts a bit more done than they like their steaks, at least in my experience. Anyway, maybe cook at 135F or so. Presear as above, then bag it in a 2 gallon Ziplock freezer and let it hang out for four hours (or a few more) in the bath. Pull it out, blot it dry, and let it sit on the counter to cool down a good long while.
While sous vide uniform doneness may be appealing (to some) on a steak, it's weird on a 4kg rib roast. I think people really like a bit of a gradient on the finished product, with some crunchy well done bits on the exterior. That's why I let my sous vide roasts cool off significantly before I do the final oven sear. This also helps prevent totally overcooking things during the final sear. Anyway, after it's rested for a while and your convection oven is cranked to 11 and soaked in pretherms, throw in the roast and pull it when it looks optimally delicious. Alternatively, you could do your final sear/cook over wood or on a grill or something to impart some of those smokier flavor notes. I've done this method for several family holidays, and it's a super easy crowd pleaser.