in parallel
dvc 2 ohm is .5 ohm
dvc 4 ohm is 1 ohm
in series
dvc 2 ohm is 8 ohm
dvc 4 ohm is 16 ohm
I don't know where you got this information, but it appears incorrect, unless you are meaning for 2 dual voice coil woofers (4 voice coils total), your information would be correct for 4 total voice coils (either 4 single voice coil woofers or 2 dual voice coil woofers).
In parallel
2 x 2ohm voice coils = 1ohm. 4 x 2ohm voice coils = .5ohm
2 x 4ohm voice coils = 2ohm. 4 x 4ohm voice coils = 1ohm
In series
2 x 2ohm voice coils = 4ohm. 4 x 2ohm voice coils = 8ohm
2 x 4ohm voice coils = 8ohm. 4 x 4ohm voice coils = 16ohm