TPControl now supports downloading the TP4 file by logging into you TPCloud account. All you need to do is, (refer to the image below) click on the "Files" tab and choose the desired TP4 that needs to be downloaded. Under the "View Details" tab, click on the green "Download" button to proceed downloading the TP4 file.
The obsolete method:
Alternatively, retrieving the altered TP4 file for Apple devices may be possible using a utility called iMazing (http://imazing.com/?diskaid=1), formerly called DiskAid.
Now, if you are successful in retrieving the file, do not make and save any changes within TPDesign4 for that file - it will corrupt. Rather, you should be able to load the file in TPDesign4, but you will need to create a separate TP4 file and copy/paste pages and popups to the new file in order to re-create an editable version of the original TP4 file.
The following directions were applicable to Diskaid, but may still be applicable for the iMazing application:
Ensure that within 'DiskAid -> Preferences -> Storage' you have enabled the "Display all Apps even if they don't have their 'File Sharing' enabled", "Show root path for Apps", and "Show hidden files and folders" options. Then connect the device to browse the file storage under 'Apps'. If available, the file will be listed under 'Apps -> TPControl -> Documents -> Slot#' where # is the file slot number the TP4 file has been loaded into.