Many things to take into consideration there.
#1 : if documents are “old” (like 15-20 years or more) you should consider re-open them and re-save them using 4D Write. This will ensure you that your documents will been saved with a recent version of 4D Write. This worth for documents in the database (where I wish they are) as for documents on disk.
#2 : if your documents are saved in picture fields then create new blob fields next to them. Load documents from picture fields and save them into blob fields.
#3 : if your documents contain pictures using PICT format, then you MUST get a version of 4D that still supports this format (obsolete since a while), convert them into a more modern format like .png then save your documents again.
Once this is done, you can use a 64 bits version of 4D like v18 to convert the 4dwr documents (from BLOBS) to 4D Write Pro and save them as Object fields.
The most recent version of 4D you use, the better, because importing from 4D Write has been improved since the origin of 4D Write Pro. bug fixes and features.
Then finally, read the documentation about the new language and concepts of 4D Write Pro. Just remember that, if documents can be imported and are compatible, the language is not. In your case, this might be the hardest part of the migration.