In businesses with a category of mexican, how many of them has a star rating below 4?
category of mexican refers to category_name = 'Mexican'; star rating below 4 refers to stars < 4
SELECT COUNT(T1.business_id) FROM Business AS T1 INNER JOIN Business_Categories ON T1.business_id = Business_Categories.business_id INNER JOIN Categories AS T3 ON Business_Categories.category_id = T3.category_id WHERE T1.stars < 4 AND T3.category_name LIKE 'Mexican'