Four is the sum and product of two with itself: 
  
    
      
        2
      
    
    {\displaystyle 2}
   + 
  
    
      
        2
      
    
    {\displaystyle 2}
   = 
  
    
      
        4
      
    
    {\displaystyle 4}
   = 
  
    
      
        2
      
    
    {\displaystyle 2}
   x 
  
    
      
        2
      
    
    {\displaystyle 2}
  , the only number 
  
    
      
        b
      
    
    {\displaystyle b}
   such that 
  
    
      
        a
      
    
    {\displaystyle a}
   + 
  
    
      
        a
      
    
    {\displaystyle a}
   = 
  
    
      
        b
      
    
    {\displaystyle b}
   = 
  
    
      
        a
      
    
    {\displaystyle a}
   x 
  
    
      
        a
      
    
    {\displaystyle a}
  , which also makes four the smallest squared prime number 
  
    
      
        
          p
          
            2
          
        
      
    
    {\displaystyle p^{2}}
  .
In religion
Buddhism
Four Noble Truths – Dukkha, Samudaya, Nirodha, Magga
Four sights – observations which affected Prince Siddhartha deeply and made him realize the sufferings of all beings, and compelled him to begin his spiritual journey—an old man, a sick man, a dead man, and an ascetic
Four Great Elements – earth, water, fire, and wind
Four Heavenly Kings
Four Foundations of Mindfulness – contemplation of the body, contemplation of feelings, contemplation of mind, contemplation of mental objects
Four Right Exertions
Four Bases of Power
Four jhānas
Four arūpajhānas
Four Divine Abidings – loving-kindness, compassion, sympathetic joy, and equanimity
Four stages of enlightenment – stream-enterer, once-returner, non-returner, and arahant
Four main pilgrimage sites – Lumbini, Bodh Gaya, Sarnath, and Kusinara

Judeo-Christian symbolism
The Tetragrammaton is the four-letter name of God. Ezekiel has a vision of four living creatures: a man, a lion, an ox, and an eagle. The four Matriarchs (foremothers) of Judaism are Sarah, Rebekah, Leah, and Rachel. The Four Species (lulav, hadass, aravah and etrog) are taken as one of the mitzvot on the Jewish holiday of Sukkot. (Judaism)
The Four Cups of Wine to drink on the Jewish holiday of Passover. (Judaism)
The Four Questions to be asked on the Jewish holiday of Passover. (Judaism)
The Four Sons to be dealt with on the Jewish holiday of Passover. (Judaism)
The Four Expressions of Redemption to be said on the Jewish holiday of Passover. (Judaism)
The four Gospels: Matthew, Mark, Luke, and John. (Christianity)
The Four Horsemen of the Apocalypse ride in the Book of Revelation.