ABC News’ “20/20” finds itself in unfamiliar — but welcome — Q4 territory: at the top of the Nielsen ratings sheet. The ABC newsmagazine program is beating NBC News’ “Dateline” across the board (so, total viewers, adults 18-49 and adults 25-54) in fourth-quarter ratings for the first time in six years.ABC’s “20/20” is averaging 4.128 million total viewers vs.