It sounds like you're referring to calculating the "content_hash" value for a file in Dropbox as returned by the Dropbox API. You can find documentation and example code for this on the Content Hash page.
In order to calculate the value correctly, you need to follow the instructions there exactly. That includes always dividing the file into 4 MB pieces. For files smaller than 4 MB, the one (first/last) piece will be smaller than 4 MB. That's acceptable, and your code should proceed normally in that case, following the instructions to compute the value.
We love to learn from the educators who use Dropbox. Whether you teach kids, teens, adults or a combination of all three, we want to know what apps and integrations you use with Dropbox to help with teaching. Which of the ones below is your favorite, or most used tool?