OK. I've added "-4", saved it, and tried again to disable SpamAssassin on Tools & settings > Spam filter, and unmarking "Switch on server-wide SpamAssassin spam filtering" and "Apply individual settings to spam filtering". Then, the same error message:
ERROR: PleskUtilException
spammng failed: server socket setup failed, retry 1: spamd: could not create IO::Socket::INET6 socket on [::1]:783: Cannot assign requested address
server socket setup failed, retry 2: spamd: could not create IO::Socket::INET6 socket on [127.0.0.1]:783: Address already in use
server socket setup failed, retry 3: spamd: could not create IO::Socket::INET6 socket on [127.0.0.1]:783: Address already in use
server socket setup failed, retry 4: spamd: could not create IO::Socket::INET6 socket on [127.0.0.1]:783: Address already in use
server socket setup failed, retry 5: spamd: could not create IO::Socket::INET6 socket on [127.0.0.1]:783: Address already in use
server socket setup failed, retry 6: spamd: could not create IO::Socket::INET6 socket on [127.0.0.1]:783: Address already in use
server socket setup failed, retry 7: spamd: could not create IO::Socket::INET6 socket on [127.0.0.1]:783: Address already in use
server socket setup failed, retry 8: spamd: could not create IO::Socket::INET6 socket on [127.0.0.1]:783: Address already in use
server socket setup failed, retry 9: spamd: could not create IO::Socket::INET6 socket on [127.0.0.1]:783: Address already in use
spamd: could not create IO::Socket::INET6 socket on [127.0.0.1]:783: Address already in use
invoke-rc.d: initscript spamassassin, action "restart" failed.
ERROR:spammng:Unable to restart SpamAssassin service
Now, when i run again "/etc/default/spamassassin" the "-4" disappears