The correct order of decreasing basic strength of the following amine is

I….. l.p is not involved in resonance --- most Basic  I, III, IV ….. l. p is involved in the resonance…. Less basic $-{\mathrm{NO}}_2(-I,-M)\dots \downarrow$  Basic strength. 

$-C{H}_3\left(+I+H\right)\uparrow$ Basic strength.

In compound, I, lone pairs are not involved in resonance, so basic strength is highest. In compound IV, because of present of -Me group, its basic strength is high. In compound II, there is only one -NH₂ group whose lone pairs involved in resonance, hence its basic strength less than compound IV. At last, because of presence of -NO₂ group, basic strength of III is the lowest.