Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE visitor (age INTEGER, Level_of_membership VARCHAR)

### Question ###
What is the average age of the visitors whose membership level is not higher than 4?

### Accurate SQL ###

SELECT AVG(age) FROM visitor WHERE Level_of_membership <= 4