CREATE TABLE table_name_4 (laps INTEGER, grid VARCHAR(255), manufacturer VARCHAR(255), time_retired VARCHAR(255))
What is the lowest laps that has a manufacturer of yamaha, and a time/retired of +1:08.312, and a grid less than 20?
    
SQL: SELECT MIN(laps) FROM table_name_4 WHERE manufacturer = "yamaha" AND time_retired = "+1:08.312" AND grid < 20