i would like to add two coments:
1. check the upper bones, in the back legs; i am not sure that they are conected in the right place to the ass. and if the conection is correct, you have a problem with the influance of this bone on the skin (the influance should be expand).
2. i think the front legs should be lifted higher (check maibridg books)
but still a 4 legd animation is hard