will your 4-2-1 design really be that different in length though? also, if i remember right, dc motorsports initial header designs were to be 4-2-1 but the wound up going 4-1 instead (like trial and trd)...i don't know for a fact but i would assume they changed to this because the gains were better.
Yes, it should be about that long, maybe a few inches shorter, maybe a few inches longer (not sure how long the final collector will be, or how long the Toda header is). TRD
/Trial/DC made their headers like OEM most likely for sales and CARB approval. It's much easier and cheaper to just install a replacement header, and it cuts down on design research, since all those companies really did was use larger piping and smoother bends while sticking to the same design as stock (TRD
even used a stock header flange). It also keeps owners from having to modify their midpipe (and altering the stock cat [CARB part of the equation]), which would attract more customers than a header that needed custom work. TPR has taken it a step further by offering a midpipe, including a CARB approved replacement cat and high flow resonator, that fits their header to solve the problem. This does not make the midpipe CARB certified, but it is legal to replace your cat with it after a certain amount of miles or years, depending on your state.
BTW, the stock header is actually a kind of 4-2-1 design (called a theta pipe), but instead of using individual pipes for the secondaries, they just put a divider plate down the middle of the collector and down a bit of the midpipe to simulate a true 4-2-1 design. Toyota claims it was to get the cat to heat up faster on cold startup, but I've been told by a header designer that that's BS, and it was just done to reduce production costs (which is totally believable).