SOP-4 Package Footprint Standard
Many IC manufacturers use non-standard footprints - or make up their own.
Whether or not SOP-4, or any package for that mater, is a multi-manufacturer standard package is somewhat irrelevant. You should always follow the manufacturers datasheet and recommended footprint, even for more standard packages.
As to where to get the footprint, you either need to make it yourself for whichever tool you use (for something this simple, you could probably duplicate and modify an existing package), or find a CAD footprint from the manufacturer or tools such as UltraLibrarian.
If you find a CAD footprint from a secondary source such as vendor (e.g. Digikey), always double check that it matches the parts datasheet. If there are discrepancies, trust the parts datasheet.