Keep in mind that Speed Graphics and similar press cameras were used in their heyday for just the type of group shots that you had problems with, using B/W films slower than what we have available, with good success. But powerful flashbulbs were used back then, in order to properly illuminate the scene. The nice thing about bulbs is lots of light output in a compact, lightweight package. Today, you'd have to settle for a multiple head flash setup for such a shot. I think the problem isn't your camera, it's not enough light.
~Joe
Actually, you don't need to settle for a multiple head flash. Not all the good flash bulb handle flashes have been converted for Luke Skywalker. They still exist and you can still get flash bulbs, either from ebay or from a couple of companies in Europe (see comment #30 on this thread -
http://www.rangefinderforum.com/forums/showthread.php?t=87421&page=2 ) For bulb flash you either need a lens with M sync, or sufficient darkness to use the B or T setting. Bulbs tend to give a different look as well as better coverage.
4x5 is a little heavier, and you will find changing cut film holders a bit of an inconvenience (but doable). A grafmatic back will help, but keep some cut film holders for changing to a different ISO or from color to b/w and back again. For real dark, Kalart actually made some RF adapters with lights to let you know when you were focused. However, they are really hard to find.
If you want the big negative, but not quite as much weight, consider a 9x12 folder. Even with an RF they are a little lighter. You may need to clean the shutter, you may need to adjust the RF. You will find it harder to find cut film holders. All I have seen are metal, so you tend to lose some of the weight advantage of the camera to the holders. I have never seen a grafmatic for 9x12, but they may exist. You can get 6x9 and prbably 6x7 roll film holders for them. Not many of them had flash connections for the shutter. If you are handy, the IBSOR shutters lend themselves to home adaption (in fact, I have one on an RF VAG that was orginally made with a flash connection). I know some 9x12 were made to interchange lenses, but the one I have, I haven't come across any other lenses for it yet.
I like the 9x12 for weight and overall size, but I would really suggest you stay with the 4x5 and bulb flash. Much more versatility.