Writing 4,000 words in a day was a remarkable achievement for a novelist. However, other factors needed to be taken into consideration. First of all, he had to see if the novel had been conceived and had written many drafts or outlines. If that was the case, then writing 4,000 words might only be a small paragraph or chapter. However, if the novel still needed to be constantly revised and perfected, then writing 4,000 words might be a very long writing cycle that would consume a lot of time and energy. Secondly, it depended on the genre and theme of the novel. Some genre and subject matter may require more words, such as fantasy, science fiction, fantasy, etc. Some short stories may only require a few thousand words. Lastly, he had to consider the reader's reading speed and reading needs. If the reader has a high word count requirement, then more content may be needed. However, if the reader is very fast, then 4,000 words may be able to be read and understood quickly. Therefore, whether or not a 4,000-word novel was written in a day depended on many different factors. But no matter what, this is an amazing achievement. Congratulations!