Lens Mathmatics for dummies

I notice many people her have a good understanding of the mathematical/ scientific concept of digital photography. Now as being fairly new to slr photography, I'd like to get an understanding of 4/3 lens with reference to f stop.

As we know to maintain exposure if we step down our aperature we must increase our shutter speed, proportionally.

I also have read how with 4/3 there seems to be a greater dof at a given f stop than 2/3 or 135 sensors. the question I have is does the 35-100 f2 in effective terminology become 70-200 f2, 70-200 f4( since you double one do you halve the other?), or something in between?

Sorry if I ran on a bit. I'm not looking for all the technical aspects, just a general understanding.