1, MAKE THE BROTH
Put the chicken pieces, salt, and the 4 quarts water into a stockpot (about 12-quart capacity) and bring to a boil over high heat. Lower the heat to a gentle simmer and then use a ladle or large, shallow spoon to skim off any scum that rises to the top. Add the onion and continue to simmer for 10 minutes.
2, Remove the breast from the pot and set aside in a bowl of cold water for 5 minutes to prevent it from drying out. Add the pork bones, squid, and fish sauce to the pot. Raise the heat to high to return to a boil and then lower the heat to simmer gently. Again, skim off any scum. Simmer, uncovered, for 2 hours.
3, When the breast has finished soaking, drain the water and set the breast aside. Allow it to cool completely, then shred the meat with your fingers into fine pieces about ⅛ inch wide, pulling it along its natural grain and discarding the bones and skin. Put the chicken shreds in a small container, cover, and refrigerate.
4, To rehydrate the dried shrimp, put them in a small saucepan with the 1½ cups water, bring to a simmer over medium heat, and cook for 10 minutes, or until slightly soft. Drain the shrimp into a sieve placed over a bowl. Add the cooking liquid to the simmering broth. Let the shrimp cool completely, then put it into a food processor or electric mini-chopper and process to grind to a fine texture. Transfer to a small container, cover, and refrigerate.
5, When the broth is ready, use tongs to transfer the pork bones to a large bowl filled with cold water. Let them soak for 5 minutes to prevent them from drying out and turning dark. Drain the pork bones, let cool until they can be handled, and then remove the meat, discarding the bones and any odd bits. Use your fingers to break the meat into pea-sized pieces or tear it into fine shreds. Put in a small container, cover, and refrigerate.
6, Position a fine-mesh sieve (or a coarse-mesh sieve lined with cheesecloth) over a pot and gently ladle the broth into the sieve. Discard the solids, including the chicken parts. (This seems wasteful, but these parts are spent.) Use a ladle to skim as much fat from the top of the broth as you like. (To make this task easier, you can cool the broth, refrigerate overnight, lift off the solidified fat, and then reheat before continuing.) There should be about 3 quarts (12 cups) broth.
7, ASSEMBLE THE BOWLS
Bring the broth to a simmer over medium heat while you are assembling the bowls. At the same time, bring a large pot of water to a rolling boil, for reheating the noodles. Make sure the chicken, shrimp, pork, egg, and sausage are at room temperature; ready the Vietnamese coriander and pepper for assembling the bowls; and put the shrimp sauce, preserved radish, and chiles on the table.
8, Place a portion of the noodles on a large vertical-handle strainer (or mesh sieve) and dunk the noodles in the boiling water. After 5 to 10 seconds, pull the strainer from the water, letting the water drain back into the pot. Empty the noodles into a bowl and repeat with the remaining portions, while proceeding to assemble each bowl as the noodles are reheating and draining.
Visually divide up each bowl into quadrants. Cover 1 quadrant with chicken, the next quadrant with egg, and the third one with sausage. If you have less shrimp and pork than the other ingredients, fill the remaining quadrant with half of each; if you have lots of pork, cover the quadrant with it, and put the shrimp in the center. Put some Vietnamese coriander in the middle, and then sprinkle with pepper.
9, Raise the heat on the broth and bring to a rolling boil. Do a final taste test, adding more salt, if necessary. Ladle about 2 cups broth into each bowl, distributing the hot liquid evenly to warm all the ingredients. Serve immediately with the fine shrimp sauce, preserved radish, and chiles. Diners should stir in about ¼ teaspoon shrimp sauce to finish their bowls. The radish and chiles add crunch and heat.