What is the exact error message?
Cannot open the file (file name), Adobe InDesign may not support the file format, a plug-in that supports that file format may be missing, or the file may be open in another location.
That error is typical of having the file open (or ID thinking the file is open because the .idlk was not deleted on close), or trying to open a file that isn't really a .indd, .indt or .idml file (the formats that ID can open). Check the folder where the file is located for a .idlk file and delete it if you find one.
1. Super appreciate your help.
2. It is saved on my desktop - there are no .idlk files
3. I sent the file to another person and they cannot open it
4. The concern is that it is a 4 GB file - which is not normal for an unpackaged file - does that not seem large? Generally even the most populated of docs is in the MB category
5. All other files open fine
6. I copied and pasted under a new name, rebooted a billion times
4 GB is a huge file, and it may well be corrupt. Can you open in a text editor and show us the first couple of lines of what you see?