4-season quilt?

You would roast inside a 4 Season quilt in summer.

If I had to buy only one quilt for four season in use in SC it would be an Enlightened Equipment 20 degree down quilt. Then I would buy a 50 degree Synthetic quilt in the summer when you realize it's not a good idea tho only have quilt.

It depends on the difference between your minimum and maximum expected temps .
I use the same sleeping bag from about 68f (fully open ,just underpants on,low R mat) to 20f (socks/gloves/hat/merino top bottom, puffy jacket and pants, DM7 mat) however it would not work below about that temp.

I switched to quilts this past winter, and use a 50F with a snap/drawstring footbox when it's warm, and an underrated "30F"(more like 20F, at least for me) with a closed footbox for cooler weather.
The 30F is a Katabatic Palisade, worked down to 0 during the polar vortex with grid fleece pants and a down jacket over my baselayers, and is still perfectly comfortable at 40-50F.
The 50F is an Underground Quilts Flight Jacket, and hasn't seen use on the warmest of nights this year, but I sleep in the thing at home with the thermostat set on 74 with the footbox unsnapped and the drawstring cinched down at the bottom just enough to keep it around my feet.
They make a good combo. If it's too cold for the 50, you're into temps where you're comfortable in the 20, and vice versa.

Be realistic about what temps you'll be out in when buying, though. I'm out regularly year-round, but if I avoided the coldest temps in winter and warmest in summer, would probably have just bought a 40F quilt('cause I've used a 40F bag for all but the hottest and coldest temps for the last several years in AL and TN), and saved a lot of $$$.

It's easier to get warm than it is cool outdoors. I'd also suggest a 20* quilt but you're going to be toasty in the summer.

Couldn't you just sleep atop the quilt in the summer? Alternatively, has anyone tried getting a lighter quilt (30 or 40 degree) and bringing a pocket-sized space blanket in winter to use for extra warmth under the quilt?

Southeast?

Like the Smokies in the winter?

What part of Southeast are you talking about.

I was thinking of the deep south mostly: Louisiana, Mississippi, Alabama, Georgia, and perhaps even Florida. Mountains weren't really in mind, but if a system could be devised wherein a single quilt could handle mountains as well as lowlands, that would be ideal.

Thinking about it now, you could do it with an older model Enlightened Equipment with Karo baffles by moving a lot of the down to the sides.