In Gen 8:20-21, we have references to four features of later sacrifices. What are they and what significance might they have?

Indeed, four features of Mosaic-style sacrifices can be found here, which might suggest anachronism (to a semi-attentive reader), but not necessarily: the author could just as easily be offering the authority of Noah in establishing the legitimacy of certain basic features of Mosaic-style sacrifice, claiming that these were merely the divinely-ordained antecedents of practices later codified. The four features are: an altar (albeit not quite of the sort God tells Moses to make); the use of “clean beasts” and fowl, only, for sacrifice; the use of animals of various kinds for an important sacrifice (here, one of each is offered); and the sacrifices were a “sweet savour” to the Lord, a phrase frequently repeated in Leviticus in describing what sorts of sacrifices God wanted. As to the significance of these practice, first, the altar sets the victim apart, making quite clear what is in the offering. The clean beasts are mostly livestock and poultry, i.e., animals man raises—although, interestingly, some clean beasts, such as deer, though good for food, are not specified as sacrificial victims in the Mosaic code. Clean beast requirement means the animals are among those God is to give the Hebrews for food, hence they are sacrifices indeed, the giving up of food. Using multiple kinds of clean beasts, when only seven were left in the world (plus whatever young were born on the ark), indicates thankfulness for all the food animals that were saved and kept available. As to the “sweet savour,” see the next question.