Rhonda Hovan's slow growth plan for Goldens:
slowgrow
provides valuable information about target weights, exercise etc.
Here are the target weights for a Golden Retriever:
Age Weight
12 wks. 15-16 lbs.
16 wks. 22-23 lbs.
20 wks. 28-30 lbs.
While every puppy is different, 40lbs for a 4 month old is well above the target for an approximate 5 month old.
Also, 20 inches seems quite tall for a 4 month old considering the standard for a male golden adult is between 22 inches and 24 inches (+/- 1 inch). Slow growth is actually much better than a faster growth rate.
I actually measured my almost 4 month old male and he is about 15 inches at the whithers (could be off a little because I was using a tape measure). Won't know the weight until he goes in for last set of shots on Tuesday but he was 16lbs at 3 months.