When Soaking Wounds with a bennie do you roll for the maximum number of wounds the damage could do, or the maximum wound the character can take. For example, say a character currently has 2 wounds and they are hit with damage that could do 2 wounds, do they roll to Soak one or two wounds? What about 10 wounds?
I'll use your example.
The character has 2 wounds and takes 2 more. They decide to spend a benny and soak the wounds. They make a Vigor check at -2 (due to having 2 existing wounds.) If they succeed (target 4 always), they soak 1 wound. Each raise beyond that soaks another wound.
They can only soak wounds immediately after receiving them. The original 2 wounds remain until further healing can be applied, regardless of how many raises were rolled. If all immediate wounds (the latest 2 wounds) are soaked, the character also recovers from being shaken.
If a character receives 10 wounds in total, they have to somehow soak 7 wounds with one Vigor test to prevent being incapacitated (target 28, good luck!) I've been there before and come out with a permanent injury and a new quest.
It is also worth noting that once you get past 6 wounds at once, you are almost certainly better off saving your bennies for rolls on the incapacitation table rather than trying to soak the wounds themselves.