Either

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


[x(x+3) +1]2 = [(x+1)(x+2)-1]2 = (x2 +3x + 1)2


or


x(x+1)(x+2)(x+3) +1 = (x2 + x)( x2 + 5x + 6) + 1


= x4 + 6x3 + 11x2 +6x + 1


= (x2 +3x + 1)2