If you need more time, stop down to f11, or you could try ND filters.
How did you do this maximum black test:
-Assuming you had a test negative in the enlarger - developed to film base + fog density?
-Why grade 4?
Also worth noting "maximum black" is not a very useful test anyway. Not only are there many variables (film type, film development/developer, grade of paper or filter, type of paper, batch of paper, paper developer, light source stability, difficulty judging maximum black by visual inspection), but in the end it doesn't really tell you very much.