First, check your permalink settings: Dashboard -> Settings -> Permalinks
Second, check to ensure you don’t somehow have a Page slug conflict. Go to Dashboard -> Pages, click the “Trash” tab, and ensure you don’t have any Pages in the trash (if so, delete them if you don’t need them).
Third, check the page-slug for your “About-Us” Page, and ensure that it matches what is being linked in your Page Menu. If they don’t match, that will explain your 404 error.