My first guess would be the ftp server can't properly handle files larger than 4gb. Second would be that perhaps you're using FAT32 file system which is limited to 4gb.
FlashFXP is fully capable of working with files 4gb and larger.
U r right!
I tried IoftpD, adn everything was fine.
I use Serv-u, and when i wrote to serv-u's forum, the answer was: Your client doesnt support the large files. Try to use other client.