When encountering issues converting 4D Write documents to 4D Write Pro documents, checking that the 4D Write files have been successfully converted to the blob field may be helpful. To do this, use QUERY BY FORMULA in conjunction with BLOB size to verify that records in the blob field have content. For example:

QUERY BY FORMULA([Letters]; BLOB size([Letters]WPblob)>0)

Using the above code, check the current selection of [Letters] in debugger to see if there are records with non-zero blob size in [Letters]WPblob. If zero selected records are returned, that probably means the conversion was not successful (given that the 4D Write letters had content initially).