How can I get the coordinates in, for example EPSG:4326 of a specific location, in either OpenStreetMaps or Google Maps?
As far as the projections are concerned, this answer explains it really well. Once you have the coordinates in either
EPSG:3857 you should be able to covert them fairly easily, using either
PostgreSQL or online convertors like this one.
Getting the actual coordinates of a place or point in GoogleMaps is as simple as just clicking on the map. You should get a marker where you click along with a small pop-up which shows the name, coordinates of that point.