It sounds as though you are using a FAT32 hard drive and are trying to export uncompressed AVI.
4GB is the maximum file size allowed on a FAT32 drive which is why the NTFS format is preferred.
Try saving to a NTFS formatted drive if you have one (Right click the drive in File Explorer and choose Properties to find out) or use a codec like DivX or Xvid to reduce the file size. Alternatively use WMV or MP4 formats instead which will also reduce the size.
Peter
Forum Administrator
www.reallusion.com