The argument of Republic IV suggests that Plato certainly thinks that it’s possible to think something is right and not do it, and think something is wrong and do it anyway.
This is what the examples in Republic IV seem to give us. But does he think that it’s possible to know that something is right and not do it, and know that something is wrong and do it anyway.
That’s not so obvious. It’s not implausible to suppose that Plato thought that you could only strictly know what’s right and wrong if your soul were in complete harmony. But if your soul were in complete harmony, you would never have the naughty motive which would lead you to fail to do what you think is right, or to do what you think is wrong. So it looks as if Plato might think that if you really know what’s right and wrong, you’ll always do the right thing.
So perhaps he thinks that virtue is just a kind of knowledge – exactly as Socrates is said to have thought. This will depend on exactly how Plato thinks of knowledge in the central books of the Republic.