CREATE TABLE table_63543 ("Children of Earth" TEXT, "Run time" TEXT, "Writer" TEXT, "Director" TEXT, "Original airdate" TEXT, "Viewers  ( including HD )  in millions" TEXT)

Who is the writer of Children of Earth of 'day four'?
SELECT "Writer" FROM table_63543 WHERE "Children of Earth" = 'day four'
SELECT "Writer" FROM "table_63543" WHERE "Children of Earth" = 'day four'