CREATE TABLE table_17798548_4 (_number_of_discs INT, season VARCHAR)

How many discs for the complete 4th series?
SELECT MAX(_number_of_discs) FROM table_17798548_4 WHERE season = "The Complete 4th Series"
SELECT MAX("_number_of_discs") FROM "table_17798548_4" WHERE "The Complete 4th Series" = "season"