How many constitutional isomers are possible for C4H9Br?

I count four constitutional isomers of ##"C"_4"H"_9"Br"##.

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

(from hardinars.tk)

and

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

From the straight-chain isomer, we get

1-Bromobutane

and

2-Bromobutane

From the branched-chain isomer, we get

1-Bromo-2-methylpropane

and

2-Bromo-2-methylpropane

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