Commentaire sur Le Lévitique 11:21

על ארבע means UPON FOUR legs.

אשר לו כרעים. which have jointed legs above their feet, etc. In my treatise on Tur Yore Deyah 85 I have explained in detail that Rashi is correct that these winged creatures have to have legs which are attached close to their throats. People who permit these creatures to be eaten because there are no species which conform to this description are in error. The fact is that of the four, respectively eight categories of winged creatures listed in verses 22 as permitted to eat, only one category is found in our parts of the world. The eight kinds of forbidden swarming things listed in verses 29 and 30 also do not have their habitat in parts of the globe inhabited by Jews. Most of the winged things that do abound in our part of the globe do not have the kind of jointed legs that would make them edible according to the Torah's criteria. Most of these details are discussed in Chulin 63-65. The important thing is that the fact that we do not find the species mentioned in the Torah in our parts of the globe is no proof that they do not exist and that our sages have misinterpreted the Torah. Any G'd-fearing Jew will abstain therefore from eating any of these things and will protest when he sees other Jews eating them. For the last 12 years after I have published these words and people stopped eating these kinds of "grasshoppers," our land has not been afflicted by the plague of locusts. This is proof that observance of Torah and the commission of good deeds protects us against natural disasters.

לנתר, to jump, to shake, as per Onkelos. The root occurs in this sense in Chabakuk 3,6 ויתר גוים, “He makes the earth shake.” This transitive mode of the root נתר, natar, also occurs in Genesis 2,21 from the root נפל, nafal.