4O

4O or 4-O may refer to:


 * Quarto (4° or 4o), a format of a book
 * 4 Degrees and Beyond International Climate Conference, 2009
 * Quaternary sector of the economy (4o Sector)
 * Air Montenegro (IATA code: 4O), the flag carrier of Montenegro
 * Interjet (former IATA code: 4O), a former Mexican airline
 * 2007 Rally d'Italia Sardegna (4º Supermag Rally Italia Sardinia)