The 4:1 style headers are most efficient at very high RPM ranges - higher than your'e generally going to be running in a street driven car. It's an acoustic thing really - how far has the last pressure wave gone down the pipe before the exhaust valve on that same runner opens up again and sends a new one down.

The 4:2:1 style header (and the later dual downpipe stock exhaust manifolds are basically this system - which is why they actually work pretty darn well) is most efficient at rpm ranges typically used in street driving. The cylinders opposite each other in the firing order are paired up sooner - that way the other cylinder's departing pressure wave can help evacuate the other cylinder's exhaust. Then the pulses from those are paired up pretty far down away from the engine.

This is, all things considered, a somewhat modest difference, though. Either style of header is going to do a good job of not bottlenecking the engine's throughput. Hooking one up to a stock exhaust system is generally going make all that fairly pointless - depending on what sort of 'stock' system it is. But a warmed up B20 probably needs something like a 2.25" exhaust.

While the style and tubing diameter of the header does have some implications on the engine's power output, the exhaust system hooked on after the header mostly just needs to 'stay out of the way' - to not provide any resistance to flow. The diameter of the header runners and the length from the valves they rejoin can help promote and exploit exhaust gas velocity around the ports, once all that jazz is done, you just need to get the gasses out of the back end of the system with no further drama.
--
'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)