Meyer lemons add a bit of floral sweetness you don't always find in marmalades, balancing out the bitterness. The preserves make a sensational glaze for roast chicken.
Remove peel and pith from 2 Meyer and 2 regular lemons. Remove seeds and coarsely chop fruit. Thinly slice remaining 2 Meyer and 2 regular lemons; remove seeds. Combine all lemons in a medium saucepan. Add 2 cups water and bring to a boil, then reduce heat and simmer 5 minutes. Remove from heat. Press parchment directly onto surface and refrigerate at least 8 hours and up to 1 day.
Place a few small plates in freezer. Remove parchment from lemon mixture; return to a boil, then reduce heat and simmer until rinds are very tender, 15 to 20 minutes. Measure mixture (you should have about 3 cups), then return to pan. For every cup of mixture, add 1 cup sugar. Add cardamom and salt.
Bring to a boil, stirring until sugar has dissolved. Clip a candy thermometer to pan and continue to boil, stirring frequently, until rinds are translucent, mixture has a golden-amber color, and thermometer reads 220 degrees, 15 to 20 minutes. To confirm doneness, drop a spoonful of mixture on a frozen plate; if marmalade has a slight film when pushed with a finger, it's done. If it spreads out and thins immediately, continue cooking and test again after a few minutes. Transfer marmalade to airtight containers and let cool completely. Cover and refrigerate up to 1 month.