Notice that AC splits the quadrilateral into two pieces.

ABC is a triangle as well as ADC.

By the Triangle Inequality for triangle ABC, AC must be greater than 6-3=3 and less than 6+3=9. The values are between 4 and 8, inclusive.

By the Triangle Inequaliyt for triangle ADC, AC must be greater than 4-4=0 and less than 4+4=8. The values are between 1 and 7, inclusive.

Thus, the values in the intersection between the two inequalities are from 4-7 for 7-4+1=4 values that satisfy AC, namely 4, 5, 6, and 7 units.