Table schema:
CREATE TABLE table_22170495_6 (total_viewers VARCHAR, original_airing_on_channel_4 VARCHAR)

Question: How many total viewers have April 21, 2010 as the original airing on channel 4?
SQL: SELECT COUNT(total_viewers) FROM table_22170495_6 WHERE original_airing_on_channel_4 = "April 21, 2010"