What's the difference between EPSG:4326 and EPSG:900913?

They are not the same. EPSG:4326 refers to WGS 84 whereas EPSG:900913 refers to WGS84 Web Mercator. EPSG:4326 treats the earth as an ellipsoid while EPSG:900913 treats it as a sphere. This affects calculations done based on treating the map as a flat plane which is why your features got plotted on the wrong places.

EPSG:4326 uses a lat/long coordinate system. Latitudes are = 90 to -90 and Longitudes are = 180 to -180

EPSG:900913 uses an x/y axis coordinate system.