Mac OS X is UNIX based. UNIX systems restrict access to TCP/IP ports 0 through 1023 to the "root" user. Consequently, 4D Server 6.7.5 cannot serve Web pages on Port 80 by default. To allow 4D Server to do this, you must run it as the root user. You can do this from the Terminal, OS X's command line application:
sudo open "/Applications/4D Server 6.7.5"
Of course, use the actual path to 4D on your hard disk.
"sudo" is the UNIX "Super User Do" command. You will be prompted to enter the password for the Super User - which, unless you changed it, will be the same password you specified for the first user when you installed OS X.