To be honest there's no hard link between mx4/5 and postgres versions. The reason you're seeing different versions is purely coincidental: new nodes get created with newer versions of postgresql. We mention the versions to help you in case you run into any weird issues and so that you know what version of postgres your backups are.
We'll be migrating 8.4 postgresql nodes in the future to 9.x, if you have nodes running 8.4 you'll be notified. If you already want to migrate now you can file a ticket for that.