Step 1: Understand the ICBN rule.
ICBN stands for International Code of Botanical Nomenclature.
It provides rules for naming plants scientifically.
According to binomial nomenclature, every scientific name contains two parts:
\[
\text{Genus name}+\text{Species name}
\]
Step 2: Recall the correct writing style.
The genus name always starts with a capital letter.
The species name always starts with a small letter.
For example:
\[
\textit{Mangifera indica}
\]
Here, \(Mangifera\) is the genus and starts with a capital letter, while \(indica\) is the species and starts with a small letter.
Step 3: Check the given statement.
Option (4) says that genus name starts with small letter and species name starts with capital letter.
This is opposite to the correct rule.
Therefore, option (4) is the wrong statement.
Step 4: Final conclusion.
Hence, the wrong statement regarding ICBN is
\[
\boxed{\text{Genus name starts with small letter and species name starts with capital letter}}
\]