To find the total cost \( C \) using the equation \( C = 15 + 3x \), where \( x \) represents the number of shoe rentals, we need to substitute \( x \) with 4 (for four shoe rentals):
\[ C = 15 + 3(4) \]
Now, calculate \( 3(4) \):
\[ 3(4) = 12 \]
Next, add this to the flat rate of 15:
\[ C = 15 + 12 = 27 \]
Thus, the total cost for getting a lane and four shoe rentals is:
\[ \text{$27.00} \]
So, the final answer is:
\[ \boxed{27.00} \]