When it comes to prime ribs I usually go by the number rather than by the weight. Because, when I cut each slice of the rib it's quite a chunky piece of meat. Personally, I can't have more than one per meal, but I do calculate about 2 per serving. So in this case, I would say I have four ribs about 1.5 kg (4.5 pounds) but only 4 servings.