How many structural isomers are possible for $C_4{H}_{10}O$? (only alcohol)

The structural isomers (only alcohol) are as follows:
IMAGE $01$ is $butan-1-ol$
IMAGE $02$ is $butan-2-ol$
IMAGE $03$ is $2-methyl-propan-1-ol$
IMAGE $04$ is $1,1-dimethyl-propan-1-ol$