A short answer that sidesteps the question of what numbers "are" is that if two is 1+1 and four is 1+1+1+1 then it follows from the associativity of addition:

A short answer that sidesteps the question of what numbers "are" is that if two is 1+1 and four is 1+1+1+1 then it follows from the associativity of addition: 2 + 2 = (1 + 1) + (1 + 1) = ((1 + 1) + 1) + 1 = 4