Each natural number divisible by 4 is a difference of squares of two natural numbers, i.e. 4
        x
      
    
    {\displaystyle 4x}
   = 
  
    
      
        
          y
          
            2
          
        
      
    
    {\displaystyle y^{2}}
   − 
  
    
      
        
          z
          
            2
          
        
      
    
    {\displaystyle z^{2}}
  . A number is a multiple of 4 if its last two digits are a multiple of 4. For example, 1092 is a multiple of 4 because 92 = 4 × 23.
In politics
Four Freedoms: four fundamental freedoms that Franklin D. Roosevelt declared ought to be enjoyed by everyone in the world: Freedom of Speech, Freedom of Religion, Freedom from Want, Freedom from Fear. Gang of Four: Popular name for four Chinese Communist Party leaders who rose to prominence during Chinas Cultural Revolution, but were ousted in 1976 following the death of Chairman Mao Zedong. Among the four was Maos widow, Jiang Qing. Since then, many other political factions headed by four people have been called "Gangs of Four". In computing
Four bits (half a byte) are sometimes called a nibble. In science
A tetramer is an oligomer formed out of four sub-units. In astronomy
Four terrestrial (or rocky) planets in the Solar System: Mercury, Venus, Earth, and Mars. Four giant gas/ice planets in the Solar System: Jupiter, Saturn, Uranus, and Neptune. Four of Jupiters moons (the Galilean moons) are readily visible from Earth with a hobby telescope. Messier object M4, a magnitude 7.5 globular cluster in the constellation Scorpius. The Roman numeral IV stands for subgiant in the Yerkes spectral classification scheme. In biology
Four is the number of nucleobase types in DNA and RNA – adenine, guanine, cytosine, thymine (uracil in RNA). Many chordates have four feet, legs or leglike appendages (tetrapods). The mammalian heart consists of four chambers. Many mammals (Carnivora, Ungulata) use four fingers for movement. All insects with wings except flies and some others have four wings.