How many constitutional isomers are possible for the formula C4H9Cl?

There are four possible constitutional isomers of ##"C"_4"H"_9"Cl"##.

First draw all the constitutional isomers of ##"C"_4"H"_10##. You get

(from hardinars.tk)

and

Then put a ##"Cl"## atom in every possible location on each of the carbon skeletons.

From the straight-chain alkane, we get

1-Chlorobutane

and

2-Chlorobutane

From the branched-chain alkane, we get

1-Chloro-2-methylpropane

and

2-Chloro-2-methylpropane

And there are your four constitutional isomers of ##"C"_4"H"_9"Cl"##.