The decreasing order of basicity of the following amines is:

Basic strength depends upon availability of lone pairs. Greater the resonance of lone pairs lesser the basic strength. In $( II ) \text{and} ( III )$ lone pair is localised. $( III )$ is more basic due to $+ I$ effect of carbon chain whereas in $II$, negative mesomeric effect is operating. In $( I ) \text{and} ( IV )$ lone pair is delocalised, so their basic strength is less that former two amines. If $( IV )$ donates its lone pair then its aromaticity will get lost therefore, its basic strength is less than that of $I$.

Overall order of basic strength is \left(III\right) > $\left(II\right) > \left(I\right) > \left(IV\right)</p>$