Highlighting and Consolidating Quadruplets

Hi All,

I'm working with quite a large data set (>20,000 entries), consisting of two corresponding columns. The first column contains a number/letter ID which in some cases is duplicated up to six times. I would like to highlight all rows for which the ID is repeated four or more times, delete all other rows, and create a new column consisting of only one instance of each ID which in the original column was duplicated four or more times. Is there any way to accomplish this without painstakingly filtering and identifying these quadruplets manually?

I'm quite a beginner with Excel, so any and all suggestions would be greatly appreciated. I'm currently working with Excel 2008 for Mac -- however, if necessary, I could import the data into Excel 2007 for Windows.

Cheers,

James