f(x)=(x-4)(x-1)(x-3)(x-2)=(x²-5x+4)(x²-5x+6)=(x²-5x+5)²-1
f′(x)=2(x²-5x+5)=2x²-10x+10
Δ=100-4×2×10=20>0
∴有两个实数根