Virus and Daz and maybe Sixus have 4-armed characters IIRC. The real way to do it is to have a new geometry file with all the groups properly welded and boned, but that's a tedious job, doing all the adjustments, renaming all the 3rd and 4th arm sections (collar to fingertips), new IK for the 3rd and 4th arms, etc. It's much quicker to parent the invisible body to the hip (or the visible body). Then all you have to do is postwork out all the places where the 3rd and 4th arms look bad on the visible collars.