4x8 simply isn't big enough for 12 birds.
The Usual Guidelines say that for each adult, standard-sized hen you need:
- 4 square feet in the coop,
- 10 square feet in the run,
- 1 linear foot of roost,
- 1/4 of a nest box,
- And 1 square foot of permanent, 24/7/365 ventilation, preferably located over the birds' heads when they're sitting on the roost.
That means that 12 hens need:
- 48 square feet in the coop. 6'x8' is more practical than 4'x12' since a long, skinny coop like that would be difficult to work inside.
- 12 feet of roost
- 120 square feet in the run. 10'x12' or 8'x15' -- 8'x16' means fewer odd cuts than either of those. 6'x20' is possible, especially if your run is an open-topped, fenced area instead of fully-enclosed with a solid and/or wire roof but risks social problems because subordinate hens need to be able to pass the dominant hens at a respectful distance.
- 12 square feet of ventilation.
- 3 nest boxes.
Since you're in South Carolina, have you considered an Open Air coop -- which is a roofed run with a 3-sided shelter on the end towards the winter winds?
Alternately, you might consider a hoop coop. Both of these options are cheaper than a traditional, fully-enclosed, wooden coop.