LTL Formula Until with same atoms (aUa)

Question

I am trying to solve some past papers in preparations for my next exam and I've stumbled across the following exercise:

While I managed to solve most parts of the exercise, I am still unsure about the formula ((b)U(b)). I am unsure which node I should start checking from and on the general definition of U.

As far as I understood a state matches a general formula aUb if it satisfies the proposition b. So in this example would the states s1, s2 and s4 all satisfy ((b)U(b))? Am I approaching this the wrong way?

Thanks a lot