Basic strength depends upon availability of lone pairs. Greater the resonance of lone pairs lesser the basic strength. In (II)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)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 (III)>(II)>(I)>(IV)
