How to recover corrupted file?
The chance to recover a corrupted file is not that big, however it worth trying.
The .4ui file is a ZIP file actually, and you can use any tool that can handle ZIP file to extract the content to a directory. Inside the directory the “plot.xml” file is the one that contains most important information. If this file goes empty, there is nothing to be recovered.
If the plot.xml file is not empty, but has been truncated, you may try to recover part of the data by completing the XML data structure. You can create a new plot file and see the minimum correct xml data structure inside.
After packing the directory content into a ZIP file again, you can try loading it with ForeUI.
I think the best bet is to find if there is any backup of your plot (you may check if there is any auto-saved backup in the same directory). In the future usage, I would suggest to commit your plot to version control system like SVN or Git, which will avoid losing your data.
In this blog post, you can find an example of committing plot file to SVN automatically, after saving it to disk.