Either

Taking first and last number, or the middle two numbers:

[x (x + 3) + 1]^{2} = [(x + 1) (x + 2) - 1]^{2} = (x 2 + 3 x + 1)^{2}

x (x + 1) (x + 2) (x + 3) + 1 = (x^{2} + x) (x^{2} + 5 x + 6) + 1

= x^{4} + 6 x^{3} + 11 x^{2} + 6 x + 1

= (x^{2} + 3 x + 1)^{2}