I was looking at the Brain Cell for my soon to arrive Lenovo T400 laptop. Although the fit guide looks like it suggests that the size 4 Brain Cell, I don't think it will fit.
That is because the depth of the computer with the 4 cell battery and the 6 cell battery is not the same. The 6 cell battery is one inch deeper than the 4 cell battery which means that the depth is going to be greater than 9.4 inches.
I think there is actually an error in some of the Lenovo documentation. Here is a link to with some of the pictures of the computers with the batteries.