You can double 4 month weight. My pup just hit 20 weeks and he's 23lbs. So, that'd put him right around 46lbs. Of you can Take his current weight and divide it by current # of weeks and multiply by 52. So, 23/20 = 1.15 x 52 = 59.8lbs. Which scenario seems more likely from your experience? My vet says she thinks he will be on the smaller side given his current 23lbs.