I know a girl who just adopted a puppy, who was spayed at 4 weeks!!!! That just breaks my heart. Why?!?!
I feel that rescues/shelters should adopt out on a spay/neuter contract at 6 months. Owner has to prepay for the surgery at the shelter or their own vet. I did that with Fozzie and he still seemed way too young. I know that shelters want to make sure the puppy doesn't reproduce, but I don't feel that makes it ok to put an infant puppy through surgery and remove all of their sex hormones just weeks after birth. Screening of puppy adopters needs to be thorough just to ensure that pup is raised right and doesn't end up back in the shelter system, and that should include people who are educated and agree to get their puppy altered at 6 months. There are far worse consequences with a puppy going to an unscreened home - death, abuse, abandonment, etc.
One of the Pit puppies I fostered, my FAVORITE, was taken in to be neutered right before going to his new home at 4 months. I was very upset... he was still a little dude! And what a horribly stressful ordeal for him. Going through surgery, getting his balls removed, and going to a new home in 2 days. Ugh!!! The family he was going to was awesome, I got to know then, they lived near me and I'd completely trust them. Luckily he grew up to be healthy, just a bit feminine. I can spot an early neuter/spay from a mile away!!! I swear. They all tend to be very leggy and top heavy, with no obvious gender at first glance. The boys have tiny shriveled up weens and both sexes tend to be prolific humpers.