2.8•1.4=3.92. Pretty close to 4, eh?
The long answer is that you've increased the focal length by 1.4•, but not the entrance pupil, and that translates to a one stop loss of light at the film plane. Teleconvertors are typically either 1.4• or 2• so the result is easily calculated- one stop or two.
I've yet to use a good TC, usually you'd be better off cropping it seems. I've used Pentax branded A-series ones only, but doubt 3rd party ones would be better.