Well, I am currently fostering three, 4 week old puppies from the shelter. They were seperated from the mother and abandoned by the owner, so I don’t even know what the mother looks like. Is 4 weeks too young to estimate their future adult size?
According to my vet, the benchmark is 4 months – double their size/weight at that point.
Paw size is an admittedly non-scientific but generally accurate way to judge. If your puppies make you laugh because they stumble over their big ol’ feet, buy stock in the Purina Company.
Just a quick vote for an excellent username / title combo. 
Cite?
(I know the puppy thread rule isn’t as stringent as the kitteh thread rule but pictures must be posted!)
Besides, with a thread like this it would actually be helpful for us so that we can estimate the cuteness of the adult dogs they will grow to be. Four weeks is the technical benchmark for estimating future cuteness.
There are many growth charts out there, this one for example. Having two weights 2 to 4 weeks apart, and a general idea of breed/type, you can make a fair prediction.