I personally use apache. I wish to let php4 exist together with php5 in a single server, but what type of method maybe there is?

What I have seen hosts do would be to keep interpretation .php files as PHP4, but use .php5 for PHP5.

My host allows me put b .htaccess having a special directive to express 'process php pages within this folder and subfolders as PHP5'

Request your host, they might have something similiar.