I've not used the spreadsheet you have, but it looks to me that you don't have enought exposure on your 4 min time for the sheet to calculate a valid gradient. Note that the 4 min point on the time vs. gradient graph is way up above the over points off the graph. It looks like that point is calculated to equal 1.90 gradient. We all know it should be less than the 5.6 min gradient.
I suggest rerunning the 4 min test and give the film about 4 stops more exposure to get more steps in the wedge to have usable density.