Its true puppies all grow at different rates but I would be very suprised if he stopped growing so soon.
Poppy was around 3lbs at 4/5 months old and is now 6lbs at 8 months old so I think your baby will be larger then 4lbs.
Even though Poppy is 6lbs and isnt finished growing she is still small, I wouldnt want her any other way. They are such a worry when they are really small and she was always under foot, they can very easily be stepped on and it is a constant worry. Bigger is better
the standard is no larger then 7lbs so I expect your baby to be within the standard but more around the 5/6lb mark.