How did you install PHP 4.3? Did you download the source, compile it, and install it from there? If so, then you will have to move the default config file to the proper location and edit it from there. Try looking in /usr/local/lib for a file named 'php.ini'. If it does not exist, then look in the direcotry of the source code for a file called 'php.ini-dist' or something to that affect.
If you installed 4.3 from some premade package, then try looking in /usr/lib and /usr/local/lib for the php.ini file.
Once you have located the php.ini file, you can edit it. Near the top of the file, there is the option to turn on the register_globals.