Why does my 4 week old kitten not use its back legs properly

Question

it still looks a bit like a frog and yet its brothers are all walking well.

It is in no pain, but slips and slides along without getting its back legs underneath properly

Answer 1

 http://hubpages.com/hub/Cerebellar-Hypoplasia basically weak motor skills, these cats are disabled in effect, and though can live long lives, do have to be "sympathetically managed with affection and understanding" because they will wobble and fall, do watch the video, it could also mean your kitten/cat wouldnt have to necessarily be put to sleep.