Hi Laura,

I have also looked for a four parameter logistic program in excel. I am now using the excel Add-in IVDfit. You can add your own formula in this program, so the function you are using can be fitted too. It can do 4PL, but also 5PL fits as well as spline, point to point and polynomial regressions. Because it is in excel it may be just the program for you.
You can download a trial version at www.ivdtools.com and see for yourself