I'm not familiar with the term "core four." Which characters would you include? I'm guessing that you most likely mean Methos, Amanda, Joe, and Duncan, by fannish popularity? Those who appear most often in the opening credits sequences would be Duncan, Richie, Joe, and... tie for fourth place between Charlie and Anne and Tessa?

My own favorite seasons are the first two, so Duncan, Tessa, and Richie are my personal core that the rest of the series simply fails to respect as I feel they are due. ;-)