2 ch can be bridged into 1, ie from stereo to mono, power in theory will be doubled assuming its single voice coil 4 ohm.

if u have double voice coil sub, power will quadruple if wired in parallel 2 ohm, or quarter if wired in series 8 ohm

4ch can be bridged to 2ch, also just like above....

yes if u bridge 2ch into 1 u only get mono
if u bridge 4ch into 2, u get stereo only

yes thats why we only bridge for subwoofers, or midbass, but not for midrange and tweeters, we only bridge when we need more power, which is usually needed for bass