Here's some info from handwritten notes I took in the 1970's from a library copy of Philip H. Smith's "Scientific Design of Exhaust and Intake Systems", which I remember as being U.K.-written and oriented toward inline 4 and 6 cyl sports engines of the 50's and 60's:

For 4-cyl inline, for street and sport use with muffler(s), exhaust passages from cyls 1 and 4 should join no more than (nor lots less than) 18 inches from the valve, and the pipe they join into should be some 24 inches long before discharging into a large volume collector pipe or box that nullifies resonance effects of everything downstream from there. Cyls 2 and 3 should be dealt with the same way, and dump into the same collector box used by 1 and 4. Thus a 4 into 2 into one system is obtained, a "tri-Y".

Pipe diam from cyl head to box is to be at least the valve port diam plus 1/8 inch. The silencer may be located anywhere convenient downstream from the collector box, which is to have a volume of 12 to 15 times the volume of one cylinder of the engine. My notes don't cover the pipe size downstream of the collector box, but good sense suggests that it should be somewhat larger than the pipes from each cylinder. One thing to note is that only one cylinder is exhausting at a time, so the tailpipe doesn't carry four times the flow that any branch of the exhaust manifold carries when it's active. Same for carburetor size -- only one cyl inhales at a time.

Disclaimer: Smith's book is an old source and I have no opinions of my own from any design or experiments with such systems. The info does seem reasonably consistent with other replies and with what the aftermarket equipment suppliers offer. One could make up rearward-running extensions for the two pipes ahead of the collector box, and a shorter pipe downstream of it, to vary the box location if one is uncertain and wants to experiment. Actual results are more real than theory is.

Another U.K. book from that era is Colin Campbell's "The Sports Car Engine - Its Tuning and Modification." I have a copy and it's especially interesting on fuel mixture ratios needed by an engine at different loads and revolutions, and how SU's and Webers cater to those varying needs in completely different ways from each other.

Charles Greenlaw Sacramento, California