Four parameter self-starting function based on SSfpl

I am currently working with a self-starting function for four parameters which I based on SSfpl but with a different formula.

This is the formula for my self-starting function:

(b1 * ((b2 * x)^b4)) / (1 + ((b2 * x)^b4))^(b3 / b4)

The code below is for SSfpl with a formula A + (B - A)/(1 + exp((xmid - input)/scal)):

ir - as.vector(coef(lm(x ~ I(log(prop/(1-prop))), data = xy)))
pars - as.vector(coef(nls(y ~ cbind(1, 1/(1 + exp((xmid - x)/ exp(lscal)))), 


data = xy,


start = list(xmid = ir[1L],
lscal = log(abs(ir[2L]))),
algorithm = "plinear"))
value - c(pars[3L], pars[3L] + pars[4L], pars[1L], exp(pars[2L]))

I can't figure out how to set up the code above properly for my self-starting function. Any help is highly appreciated. Thank You.