4 (four) is a number, numeral and digit. It is the natural number following 3 and preceding 5. It is the smallest semiprime and composite number, and is considered unlucky in many East Asian cultures. In mathematics
Four is the smallest composite number, its proper divisors being 1 and 2.
Holistically, there are four elementary arithmetic operations in mathematics: addition (+), subtraction (−), multiplication (×), and division (÷); and four basic number systems, the real numbers 
  
    
      
        
          R
        
      
    
    {\displaystyle \mathbb {R} }
  , rational numbers 
  
    
      
        
          Q
        
      
    
    {\displaystyle \mathbb {Q} }
  , integers 
  
    
      
        
          Z
        
      
    
    {\displaystyle \mathbb {Z} }
  , and natural numbers 
  
    
      
        
          N
        
      
    
    {\displaystyle \mathbb {N} }
  .