* Dictionaries vary: Different dictionaries include different words, especially slang, technical terms, and proper nouns.
* What counts as a word? Do we include proper nouns (like "John")? What about abbreviations (like "NATO")? Do we consider words that are only used in specific contexts or dialects?
However, we can get an estimate:
* A rough estimate suggests there are around 40,000 words in the English language.
* If we assume that the length of words is evenly distributed, then roughly 1/10 of all words should be 4 letters long.
* This would mean approximately 4,000 4-letter words exist.
Keep in mind: This is a very rough estimate. The actual number could be higher or lower depending on the criteria you use.