A 6" pipe has a 12% greater area than 2 4" pipes. 6^2=36, 4^2*2=32. So double 4's look pretty good.

What I haven't seen mentioned, that I remember anyway, is the difference in surface area. The perimeter of a 6" pipe is 18", roughly (6*3 is good enough). For 2 4" pipes it's 24 (4*3*2). So 2 4's have 33% more area in the pipe creating drag.

Extreme example, a couple thousand 1/8" tubes would flow basically nothing at these pressures.