mroxner, I thought I'd mention that the copy of "Fresh Air Poultry Houses" republished by Norton Creek Press is the 1924 version whereas the Google copy is the 1912 version. The newer version does have a good bit more information in it. Wood's smallest recommended size of coop, as best as I recall is an 8'x12' for a "fresh air" style house that would be used in the winter.
The 3x4 coop size appears to give you the "4 feet per chicken" area that is recommended but only if...
1. - The nest boxes are raised off the floor high enough for the chickens to walk under them (someone mentioned 18"...maybe even go to 20").
2. - The dropping board is placed above the nest boxes(maybe directly on top of the next boxes) along with the roosts themselves maybe 6 inches or so above the dropping board.
3. - You place the feeder and waterer both outside the coop.
Any area that is obstructed so that a chicken can not stand there (waterer, feeder, next box, whatever) is deducted from the available "square footage per chicken".
Making a wild stab here (and figuring you will have a 4' interior height), stacking things up as stated above this will probably put the chicken's heads at around 10 inches from the roof...whether this is far enough away to keep a top-vented draft from hitting them someone else with some experience will have to say.
Going 4'x4' rather than 3'x4' will give you an extra 4 square feet for the feeder and waterer and will help to make efficient use of sheets of 4x8 plywood or other sheet goods.
It is normally recommended to have 10 square feet per chicken of run space.
It looks like you are planning your run to be 18" high, you might want to raise the coop another 6" or so to give the chickens a bit more head room under the coop...for the "porch" part it would be good to maybe build a frame so the ceiling of the "porch" will be as tall as the top of the coop...this will give the chickens a little more stretching room and will make it easier (more room) to hang their waterer outside during the warmer times of the year...could even put an outdoor roost in there.
And, just a tip....you can fence in or build more square footage into a square than you can a rectangle. A 6'x6' square run (24' stretch of wire) will enclose 36 square feet. A 4'x8' rectangular run (using the same 24' stretch of wire) will only enclose 32 square feet.
FWIW
Best wishes,
Ed
ETA: That for some reason I took it that the OP was planning on an interior height of 4 feet. Don't know where I got that from.