CREATE TABLE table_22170495_6 (total_viewers VARCHAR(255), original_airing_on_channel_4 VARCHAR(255))
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"