JUnit.org Resources for Test Driven Development

Does anyone have any recommendations on how they arrange/group their jUnit tests? Should there be one large 'test' for each method, one small 'test' for each method possibility, etc?

So, as an example... Assume the following method:

int ReportManager.createReport(Report report)

In this case, I'd want to test for the following:

Does it throw an exception if report == null?
Does it throw an exception if report == empty?
Does it throw an exception if report is duplicate?
Does it return valid ID if successful?

So given those 4 possibilities, how would you do it? Would you create a single test method (testCreateReport) that tests all 4? Or would you create four separate test methods (testCreateReportNull, testCreateReportEmpty, testCreateReportDuplicate, TestCreateReportSuccess)?

Thanks!