(X-1)(X-2)(X-3)(X-4)
=(X-1)(X-4) (X-2)(X-3)
=（x^2-5x+4)(x^2-5x+6)
=(x^2-5x)^2+10(x^2-5x)+24
=x^4-10x^3+25x^2+10x^2-50x+24
=x^4-10x^3+35x^2-50x+24