It seems likely that you managed to configure the CNAME correctly. You can test it by using
ping and ensuring that both host names report the same IP address.
$ ping developer-test.w.pw
PING developer-test.w.pw (126.96.36.199) 56(84) bytes of data.
$ ping lanparty.lsaciasi.ro
PING lanparty.lsaciasi.ro (188.8.131.52) 56(84) bytes of data.
Right now they are not resolving to the same IP for me, but my guess is that they were when you took the screenshots.
The 404 error probably indicates that the destination machine is not configured to serve content for the domain name that you are pointing at it. You would need to modify the webserver configuration of that machine and at a
virtualhost configuration for your new subdomain.