4x12 cabs are usually wired in series/parallel. That is two series pairs wired in parallel... So, take 2 8 ohm speakers wired in series for 8x2 =16ohms, now add another pair in parallel and you get 16/2=8 ohms. Really, this is much harder to type than understand. I have no electronics background beyond A level physics. I worked this stuff out from basic principles while dealing with 4x12 cabs with blown speakers and trying to work out which hole to put the little jumper pin in on my AC50/AC30 heads

The short version is if you have a 4x12 cab filled with 16ohm speakers it will be 16 ohms. If 8 ohm speakers, 8 ohms. Manufacturers often confuse the issue these days by offering stereo wiring.