taken from a forum post ...
"I have been running backup over Ubuntu, to a disk mounted via FTP. While ftp will only report a file size of 4GB, it successfully uploads files in excess of 150GB"
This might explain why the FTP server never actually timed out. Even though it only reported the 4gb ISO it was actually transferring the file. Thus it would not be "inactive".
Did you actually try to FTP the files even though you only saw them remotely as 3.9?