The cost depends on the quantity purchased. If Banderas buys 6 bars or less, the cost is $9 per bar. Therefore, for 4 bars, the cost would be \(4 \times $9 = $36\).
So, the wholesale price per bar for Banderas, if it purchases 4 bars, is \( \frac{$36}{4} = $9 \).
The correct answer is B. $9.