Depending on how many people are in the group, you can even do two semicircles, one in front, one in back. I think you mentioned four, and certainly you could experiment with this kind of setup, though I don't know if it would be preferable in your situation or not. Not something you'd likely want to do for live performance, but for a living room recording situation, it might be useful. Just keep away from the sides or strange things will happen. Also, the stereo image of the "back" semicircle will be reversed. So, if you were in a hypothetical audience, looking at the back of the "back" semicircle and could see the "front" semicircle facing you, that "front" group will image as expected: Performers on your left will be on the left. But for the "back" semicircle who have their backs to you, performers on your left will end up on the right side of the stereo image. So, front-right and left-back will mix together, and front-left and back-right will mix together.
About a month ago I did a brief session for a small a cappella men's ensemble (one of a couple that exist within the Men's Glee Club at a local university that I also frequently record). I used exactly this technique and was pleased with the results. Two AT4050s in Blumlein>DAV BG1>Mtyek 192>R-44. It was 12 guys, and I had them divide into two groups of six that we arranged in arcs in the front and rear of the mics. It was good for them, too, as they could see and hear each other more clearly than a more traditional setup would allow, and it let me get the up-close "pop" sound they wanted.
Anyway, here's one track if you want to have a listen (it's a goofy song):
http://rapidshare.com/files/251037589/Steal_My_Kisses.mp3This ended up not being the take we used, as I felt like the center is a little deemphasized compared to the sides, though having the basses on each side (by design) in this tune was going to give that impression to some degree, anyway. While this leaves a little more "space" for the soloist, it was a little too empty otherwise for my taste, and I had the center of the arcs step in just slightly and was happier with that sound.