Question: Can you tell me why my grandfather who graduated High School is only listed as 4th grade on the Census of 1940?
Answer: Your grandfather's entry you sent me reflects "H-4" which is four years of High School. At the bottom of the census page you will find a key to the numbers:
HIGHEST GRADE OF SCHOOL COMPLETED:
None = 0
Elementary School, 1st to 8th grade = 1,2, etc., to 8.
High School, 1st to 4th year = H-1, H-2, H-3, H-4.
College, 1st to 4th year = C-1, C-2, C-3, C-4
College, 5th or subsequent year = C-5