Pulse effect scavenging is exactly what you want at each collection point. More so if you're 4-1, the pipes into the collector should be in firing order sequence to create a vacuum drawing the dirty air and helping to pull via vacuum the clean air into the chamber. The inlet side operates in a similar way with pulse reversion back up the inlet tracts occurring when the valves close and the air (traveling at not an inconsiderable speed) hits the back of the closed valve and bounces back up the tract. Also why you sometimes see the plenum (if the plenum inlet is at the front) larger at the rear, to provide a reservoir else you can lower the effective pressure inside the plenum and "run out of air" so to speak.

A 4-1 is less effective at scavenging as the collection occurs only at one point so you have only one opportunity for the vacuum to occur where as 4-2-1 allows collection at multiple points. By the time gases reach the Y it's pretty much over.

Custom made headers 4-2-1.

Custom tuned length 4-1