Evaluate at a particular set of parameters the 4-parameter log-logistic function.
loglogistic4_fn(x, theta)Numeric vector of the same length of x with the values of the
log-logistic function.
numeric vector at which the function is to be evaluated.
numeric vector with the four parameters in the form
c(alpha, delta, eta, phi).
The 4-parameter log-logistic function f(x; theta) is defined here as
g(x; theta) = x^eta / (x^eta + phi^eta)
f(x; theta) = alpha + delta g(x; theta)
where x >= 0, theta = c(alpha, delta, eta, phi), eta > 0, and
phi > 0.