Move app data (on /data/data) to SD card
First, when they say there is 4 GB storage, they count all partitions, so there is 4 GB on your phone.
To move data to an SD card, you will have to use an app called Link2SD (root required) or use an adoptable storage feature (Android 6+ only). In both cases, you don't move folders manually if you do that you will probably screw up something.
Also, if you really need a Facebook app, use the lite version, or access it via a browser. The normal version uses too much space and too many resources. Generally, I say this because I assume you don't have more than 768 MB of RAM, so use it wisely.
Check out the Link2SD's tag wiki