BQ40Z50: Purpose of FET in Reference Schematic

Hi There,

I am developing a system using the BQ40Z50 gas gauge and I do not understand the purpose of one of the FETs in the Development Board Schematic.

I am looking at the schematic on page 17 of this document: http://www.ti.com/lit/ug/sluuav7b/sluuav7b.pdf

My question concerns Q4, what purpose does it serve, I can't figure it out.

Thanks,

David