U4
- 4U (disambiguation)
- Aero Commander (aircraft)
- Buddha Air
- German submarine U-4
- Gulfstream IV
- Haplogroup U4 (mtDNA)
- ICD-10
- London Buses route U4
- Maple League of Universities
- PMTair
- U-47700
- U4 (Berlin U-Bahn)
- U4 (Frankfurt U-Bahn)
- U4 (Hamburg U-Bahn)
- U4 (Munich U-Bahn)
- U4 (Vienna U-Bahn)
- U4 spliceosomal RNA
- Unemployment