I am unable to test glFTPD but I don't see why it wouldn't work.
You could try using wireshark
http://www.wireshark.org/ to analyze the connection to see what data is being sent/received and perhaps compare the results with v3, without having your exact configuration, proxy, ftp server, its going to be extremely difficult for me to determine the exact cause.
I will continue to investigate this issue but at the moment I've hit a dead end.