The structural isomers of chlorobutane are: 1-chlorobutane, 2-chlorobutane, 1-chloro-2-methylpropane, and 2-chloro-2-methylpropane; in total, there are four structural isomers of chlorobutane. 

Explanation:

• 1-chlorobutane: Chlorine is attached to the first carbon of a straight carbon chain. 
• 2-chlorobutane: Chlorine is attached to the second carbon of a straight carbon chain. 
• 1-chloro-2-methylpropane: A methyl group is attached to the second carbon of a propane chain, with chlorine on the first carbon. 
• 2-chloro-2-methylpropane: Chlorine is attached to a carbon that is also connected to two methyl groups.