K_a is more ${\mathrm{p}}^{{\mathrm{k}}_{\mathrm{a}}}$ is less and acidic strength is more.

I,II are aromatic. After removing proton negative charge is resonance stabilised. Nitrogen is more electronegative than carbon. In II negative charge is more stabilized on N-atom

In IV after removing proton aromaticity is developed.

In III less resonance 

Acidic strength (k_a) : II>I>IV>III

${\mathrm{p}}^{{\mathrm{k}}_{\mathrm{a}}}$ order: III>IV>I>II