If you specifically need 4.21 and have previously "purchased" it (even for $0), it may appear in your on the Daz 3D website under My Account.
To get Daz Studio 4.21 or the latest equivalent, you must use the official Daz 3D account system and management tools. daz studio 421 download link
To download and "put together" Daz Studio 4.21 , you must access it through your official Daz 3D Product Library . Daz 3D typically only offers the most recent version If you specifically need 4