Standard AVI files use 32-bit pointers so 4gigs is the limit. For many years, however, there has been an extension to AVI called OpenDML which allows for a virtually unlimited file size (well limited to your disk/file system).
So far as I can tell, Speedgrade does not write OpenDML AVI files, and it does not create multiple spanned AVI files to account for the 4GB limit. It just writes into the file in some arbitrary way and corrupts resulting file.
I suggest exporting to uncompressed MOV or (if you're really weary of some MOV related issues) a DPX image sequence. If you must have AVI, I would export the video in chunks or convert DPX to AVI using something like the command line program FFMPEG.