I also found this worked well to fit a 4PL for my competitive EIA:
http://www.myassays.com/four-parameter-logistic-curve-percent-bound-versus-pg-ml.assay
Hi,
Take a look at http://www.myassays.com/four-parameter-fit.assay
From here you can do 4PL, without needing to download or buy any software. Also, its preconfigured for microplate layouts.