4-cell, 6-cell, 9-cell. Do those numbers directly reflect their relative capacities. So, for the sake of the argument, if the 6-cell lasts 6 hours, would the 4-cell last 4 hours and the 9-cell last 9 hours under the same conditions?

I fancy the 4-cell for the low weight and flush fit. How much battery life will I sacrifice, relative to the bigger ones?

Does anyone have mAh or Wh figures for these batteries?

No they don't correlate to length.
No single battery can give you 9 hours ever. Unless you turn off everything and use your laptop in the dark.

Normal use 4 cell is around 2-3 hours.
6 cell is like 4 hours.
And 9 cell is around 7 hours.

This is from personal experience. If you turn off everything and lower your brightness to 1, then you will get more battery, but that's not really practical.

Some laptops with ULV CPUs can do insane battery life, but x220 is not one of them.
X301 can do 10 hours easy on 9 cells.