Welcome to JustAnswer! I am a licensed veterinarian and would be glad to help!
1) What color are the gums?
2) Are there any other signs (appetite? vomiting? stool?)
3) Are the front legs normal?
4) Are both back legs equally affected?
While orthopedic / hip disease is always on our mind - you raise a concerning piece of information - gums that are more pale than expected.WIth pale gums, that can mean bleeding or anemia, which can lead to weakness, and then as you see - hind limb (commonly) difficulty.
Pale gums can be from hemolysis (breakdown) of red blood cells such as in hemolytic anemia or even toxins such as rat poison.
Since I can not examine your dog, this is only based on your description of the gums, but would be a concern as I was suspicious of why only a 4 year old dog would have hip disease.
For this reason - I would get to your veterinarian as soon as possible, when they open - or even to the local ER if your vet is not open.
If they believe the gums are pale as well - blood tests would be needed.
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