4．已知定义在[−1[-1[−1，3]3]3]上的函数f(x)f(x)f(x)满足对于任意的x1x_{1}x1，x2∈[−1x_{2}\in [-1x2∈[−1，3]3]3]，且x1≠x2x_{1}\ne x_{2}x1=x2，都有[f(x1)−f(x2)](x1−x2)<0[f(x_{1})-f(x_{2})](x_{1}-x_{2})<0[f(x1)−f(x2)](x1−x2)<0，则不等式f(1−2x)⩾f(x+1)f(1-2x)\geqslant f(x+1)f(1−2x)⩾f(x+1)的解集为((( )))