As we know the electron releasing groups(ERG) reduces the acidic strength of the compound (via inductive effect) whereas the electron withdrawing group(EWG) will increase the acidic strength of compound, and methoxy group is ERG, and nitro group is EWG, Increasing number of EWG will increase the effect and acidity too. Hence final order will be Methoxy benzoic acid< Benzoic acid<4-Nitrobenzoic acid<3,4-Dinitrobenzoic acid.