I want to use Non-linear regression analyse for estimate strength of concrete ..........

I have some data with 4 independent variable ( Xi) and one dependent variable concrete strength ( Yi ) ..........

I used this equation Y=a1*x1^b1+a2*x2^b2+a3*x3^b3+a4*x2^b4 and found Coefficients ( a1,a2 a3 a4 ) and ( b1,b2,b3,b4) By SPSS ..........

Is my equation wrong for Non-linear regression analyse ?