CSS-JS-Booster absolutely requires PHP 5. No PHP 4, sorry... Version-wise it is tested up until PHP 5.3.
wp-content/and CHMOD it to 0777 (give it write-permissions)
wp-content/plugins/booster/htaccess/.htaccesswere put into/appended to the .htaccess-file in the root of your WordPress-site. If not append them by hand.
CSS-JS-Booster may in rare cases break some other plugins. I noticed for example that plugins trying to calculate file-paths based on the src-attribute of the script-tag break. So you need to check yourself.
If your site has many visitors, we suggest that you combine CSS-JS-Booster with WP-Super-Cache.