How does time work if the universe is the surface of a hypersphere

According to observations, the unuverse seems to be closed, and just the surface of a 4d shape. Given that the 4th dimension is time, how can there be a passage of time if the universe is a constant part of this sphere. For comparison, i can pick different "sheets" of a stack of paper that then makes up a cube, but it still only has 1 surface. As each sheet would be 1 moment in time, how can the surface expirence the flow of time?