With collectors, pressure waves are used to help suck exhaust from the next cylinder. This is called scavenging.
In a 4-1 collector, primary pipes should be arranged next to the next cylinder in the firing order. This creates a swirl effect in the collector where pressure waves create vacuum in the next cylinder in the firing order, thus scavenging more efficiently.
4-2 collectors are still effective, just not as pronounced, with the same effect.
As a side note... roots blowers have been used in big turbodiesel motors as exhaust scavenge pumps, making the turbos more efficient (at the time). Not always for forced induction.