The integration with xt:Commerce 4 is really easy, as the only thing you have to do is to install a special plugin to your xt:Commerce - in the admin panel. Then configure it and you are done...
The plugin works for both Post Affiliate Pro and Post Affiliate Pro.
Download the plugin
The first step is to download the plugin.
Upload the plugin
When downloaded, first unzip the plugin and then copy it to your cart directory over FTP. As you can see, after unzipping you have the directory structure exactly named the way you have to upload it to your cart files...
Activate the plugin
When uploaded to your FTP to plugins/ directory, you can login to your cart admin panel and activate the plugin in Plugins> Uninstalled plugins.
If you cannot find the plugin at Plugins > Uninstalled plugins, then check the Plugins > Installed plugins section just in case upon upload the plugin happened to be automatically installed.
When installed, navigate to Plugins> Installed plugins and configure the plugin based on instructions and examples in the plugin configuration.
You can also try out an alternative plugin by 4tfm.de
Download it from http://4tfm.de/4tfm_postaffiliatepro/