Law of multiple proportion is not applicable to:

When two elements chemically combine to give two or more compounds, the weight of one element which combines with fixed weight of other element are in a ratio of small whole number. This is law of multiple proportion.

1. In case of oxides of Nitrogen, $N_{2}O, NO, N_2{O}_3 and N{O}_2$follow law of multiple proportion
2. In case of oxides of Carbon, $C{O}_2 and CO$follow the law.
3. In case of iron oxides, $F{e}_2{O}_3, FeO and F{e}_3{O}_{4 }$follow the law.
4. Oxide of aluminium forms only $A{l}_2{O}_3$. Hence, it does not follow the law.

Hence, option 4 is correct.