The Census Bureau’s Usability Lab conducted several rounds of usability testing of the online 2011 American Community Survey Instrument. This report summarizes the finding s for rounds 4a and 4b of testing conducted in October - December 2010. We identified elements of the user-interface design that were problematic and led to ineffective and unsatisfying experiences for potential respondents of the survey. Some of these issue s included duplication of names on the roster questions, privacy and confidentiality concerns, and difficulty with the income questions. The first and second rounds of testing are documented in Ashenfelter, Holland, Quach, Nichols, and Lakhe (2011a). The third round is documented in Ashenfelter, Quach, Holland, Nichols, and Lakhe (2011b).