已知函数 f(x)=ln(1+x2−x)+1,f(a)=4f\left( x\right) = \ln \left( {\sqrt{1 + {x}^{2}} - x}\right) + 1,f\left( a\right) = 4f(x)=ln(1+x2−x)+1,f(a)=4 ，则 f(−a)=f\left( {-a}\right) =f(−a)= ___.