Bears stand up frequently to see better and to display threat, to acquire something they want, or even to rub themselves on trees. I recall a famous bear from New Jersey named Pedals that would walk on his hind legs because his front paws were burnt in a forest fire. So to answer your question, it is unlikely that it is painful given that they do stand up on their hind legs frequently if they have reason to. Bears and other such animals that walk on fours are quadrupeds, so they walk on fours as their skeletal structure favours it. Humans are bipeds, and so they walk on two by comparison.
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