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)
• GPT-4o, a large language model developed by OpenAI

See also

• Fourth (disambiguation) (4º)
• 4° (disambiguation)
• O4 (disambiguation)
• 40