Stabitlity of carbocation depends various electronic effects. Least stable carbocation among the following is

1)Electron withdrawing power of  $-{\mathrm{NO}}_2>-\mathrm{CHO}$      

EWG $\downarrow$ the stability of carbocation by intensifying the positive charge

2)Electron donating power of  $-{\mathrm{OCH}}_3>-{\mathrm{CH}}_3$

 EDG $\uparrow$ the stability of carbocation by dispersing the positive charge
$\therefore$  Stability of 1 < 2 < 4 < 3