What would happen if I deleted all my data used by Google Play Services?
Google Play services offers APIs to other apps. Data used by Play services is mostly cached data for these APIs, duplicated data of Android wear apps synched with your phone and some kind of search index. If you delete this data, chances are Google Play services will just recreate it, although 3.9 GB is really much (mine only use 300 MB). But you won't delete any of your personal data by deleting Play services data.