Need suggestions to back up why walking a 3 month old pup 4 miles is too far for the age.

Have a very nice, well meaning family who mentioned they are doing this and I told them it was too much. They are very open to changing their routine but I wanted to cite a few sources to back myself up.
Their vet told them 4 miles was just fine

I think the best way to look at it is common sense, besides, a three month old puppy doesn't have his rabies vaccine yet either. I tell puppy families that their puppy can run and play as much as they want in the yard and grass, and short walks around the neighborhood, stopping frequently to socialize.

This may help too, since it is a trainer's opinion.
http://www.tenderfoottraining.com/exercise.html

Thank you! I will send that to them.
They are going to cut back

No one else has opinions on this?

Yes, I'm quite certain many others have opinions on this. I'm also quite certain the opinion is the same. Four miles is TOO FAR for a 3 month old puppy. Think about it. Would they ask their 3-year old child to walk 4 miles on his/her own? (No stroller, of course.)

Seriously?

The other poster had a valid point. The puppy shouldn't be venturing too far out of the yard/neighborhood without all of its vaccinations.

Trainer wrote, "I agree that four miles is too far, but have you read Ian Dunbar's stuff lately? He says that socialization is paramount and that more puppies die (in shelters) because they are unsocialized than from Parvo etc. that they might pick up before they have finished all their vaccinations."

Surely moderation in all things works. Don't forget the unsocialized pups in shelters include semi ferals, pet store pups, etc. You can socialize well-raised pups with dogs that you know are healthy and vaccinated without exposing a pup to every stray and coyote in 2 to 4 miles, and avoid injury at the same time. Kids play tee-ball instead of pitching to avoid overuse injuries; puppies should avoid them, too. You can have puppy parties or go on field trips. You can go to puppy kindergarten with vaccinated dogs. There are lots of ways to socialize without risking the pups. Do vaccinate with high titer parvo vaccines for extra early immunity that should override maternal antibodies.