To find out how much time Maria has to spend on each part of the test, we first need to determine the total time available and then divide that time by the number of parts.
Maria has a total of \( \frac{3}{4} \) of an hour to complete all 4 parts of the test. To find out how much time she has for each part, we divide the total time by the number of parts:
\[ \text{Time per part} = \frac{\text{Total time}}{\text{Number of parts}} = \frac{\frac{3}{4}}{4} \]
Dividing a fraction by a whole number is the same as multiplying the fraction by the reciprocal of the whole number. Thus, we calculate:
\[ \frac{\frac{3}{4}}{4} = \frac{3}{4} \times \frac{1}{4} = \frac{3 \times 1}{4 \times 4} = \frac{3}{16} \]
Therefore, the amount of time Maria has to spend on each part is
\[ \boxed{\frac{3}{16}} \text{ of an hour}. \]