Step 1: From statement I, the ratio of the present ages of P and Q is 3:4. Let the present ages of P and Q be \( 3x \) and \( 4x \), respectively.
Step 2: From statement II, the ratio of the present ages of Q and R is 4:5. Let the present ages of Q and R be \( 4y \) and \( 5y \), respectively.
Step 3: We have two variables \( x \) and \( y \), and no further relationship is provided between \( x \) and \( y \). Therefore, we cannot determine the ratio of P’s and Q’s ages four years ago with the information given in both statements. Thus, the answer is \( \boxed{{C}} \).