Animals in Need in Little Irchester is looking after the youngsters and their mum Eve, a white bichon-poodle cross, after the heartbreaking discovery by a dog walker.
The charity was called by a vet after the dogs were found in a tub in Newport Pagnell, about 15 miles away down the A509.
The four puppies, which haven't yet been named, were not even a day old and still had their umbilical cords on. Eve was not micro-chipped.
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Kennel manager Kate Archer said: "The puppies are not in the best condition, it was cold and wet. It's going to be touch-and-go.
"I cannot understand the mindset of someone doing that to a dog."
She described Eve as "a sweet little girl' and it is believed to be her first litter. Kate said she would have found it traumatic.
It is not known if Eve was dumped before she had her puppies, after she had them or whether her owner panicked when she started giving birth.
Eve and her puppies will go into foster care for 24-hour support. After 12 weeks they could be re-homed.
Kate urged anyone who finds themselves in a similar situation to contact them for help
She added: "It's quite sad that someone could not turn to us for support.
"We are here to help."
The charity, which currently has about 50 dogs and will start re-homing again in January, is appealing for donations to help cover vet bills and supplies for foster carers.