4 week old can't stand on back legs?

Answers:
you shouldnt even have a dog until its 8weeks !
i suggest bringing it to the vet .

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I have a choc lab puppy that had problems standing on the back legs too. He needs to eat more often on the mothers milk. Are you bottle feeding? Don't give him regular milk. Buy the milk that you can get at pets mart and make sure he eats every two hours. He should get stronger after that.

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http://www.bulldoginformation.com/swimmi...
it is a known and documented dysfunction and needs to be properly managed. (see above link)

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I'd take this pup into the vets to be looked at. Sometimes the pups can be injured and need help from the vets.