Let the four consecutive natural numbers be x,(x+1),(x+2),(x+3).
Product of first and last =x(x+3)=x2+3x
Product of middle two terms =(x+1)(x+2)=x(x+2)+1(x+2)
=x2+2x+x+2
=x2+3x+2
Clearly, we can see that the difference between the product of middle two terms and the product of first and last term is equal to 2.