So, are you taking the originals out of the combo and putting them in a 4x12 with the new ones? If they are all 16 ohms you have a few options...
Option #2 : You can go parallel/series and have a single 16 ohm load cab.
Option #3 : You can use a "stereo switch" (as seen on Marshalls) and have a choice between a single 4ohm or 16ohm load.
Here's the switch... http://www.mojotone.com/item.asp?pid=634611&pg=13797&id=4114500
The cool thing about having a 4x12 with two types of speakers is you can do an "X" pattern. Since you have two V30's and two G12H's, put one of each on top and then put the opposite one on the bottom of the other. Spreads the sound out really nice....