I answered a similar question here: Any luck getting specific word count? - #3 by siavosh87
The easiest way is to include an example in your prompt, where the example contains a high quality detailed lesson with the word count that you are looking for. The downside is that your prompt will be longer and therefore cost more.
If you don’t care about the cost, then the ideal solution would be to include two good examples in your prompt, and then you should get the desired word count. If cost is an issue, then I would actually try using gpt-3.5-turbo but using a prompt where you have 3-4 really good high quality examples in your prompt of the type of output you want produced. There’s a chance that doing this with gpt-3.5-turbo may give you the same quality as gpt-4.