There are many living things that only have four letters, off the top of my head in one minute I came up with the following (only one is plural since you said "a living thing")...
ants, aunt, bird, bush, calf, crow, dame, fish, frog, goat, tree
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Envy Is A 4-Letter Word
4 letter word for drum is "Dhol".
Salt is a 4 letter word for seasoning.