The additional draw is exactly why it adds the extra 4 pins. the 4 pins add a 12v (Yellow) a 5v (Red) a 3.3v(Orange) and an additional Ground pin (Black). The adapters as you've noted simply split those from existing pins.
Of course, whether those pins are separate rails are something I'm not privvy to and probably depends on the PSU itself. Generally more recent power supplies that were made to support the 24-pin connector have increased amperage ratings on the affected rails to account for the increased draw. Older, 20-pin supplies will not have this.