There will never be a fully updated PHP4 rpm for Fedora. The Fedora philosophy is to support the latest version only. Hence the PHP4 is already discontinued. The guide linked above only references the latest PHP4 version released through the legacy support services (http://www.fedoralegacy.org/
Hence you must recompile on your own or custom configure another SRC.RPM from somewhere else to suit your needs. There really is no simple solution as your needs are not very common.
And technically yes you can have both PHP versions parallely installed. You must compile them both as different modules and loaded separately in Apache. Then you must make sure the handlers for each module are specifically designated, ie. .php4 files load with the libphp4.so module and .php files load with the libphp.so module (which would be defaulted to PHP v5 from Fedora). Or something similar.
As for running 2 versions of Apache, that is also doable, but seems wasteful - unless the configurations for each will differ singnificantly. You can simply write your own script that launches an 2nd instance of httpd with manually specifiying your own custom httpd.conf and other config files and settings which are specific to your 2nd instance of Apache.
Some other distributions like SuSE do keep PHP4 at the latest version. You could try that or you can work out all the issues in porting their SRC.RPM. However in the end you most likely will have to build from source.
My opinion, yes you are right about FedoraForum it is more novices than highly technical people. Try linuxquestions.org or an apache or php mailing list or newsgroup for more focused questions. And finally, comparisons from Windows to Linux are meaningless and your expectations from FREE support are rather high.