The Census Bureau estimates that Adams County’s population dropped below the 4000 level in 2012. It’s one of the smallest counties in Iowa, with the smallest population. Its population density was already under 10 people per square mile in the official 2010 census.
Estimates for each county are available here. If you click the “Download” icon there will be options for PDF or Excel spreadsheet. (Excel is evil, but at least Apple’s Numbers opens the files.)