GrabCAD is the largest community of professional engineers. While many files are for solid modeling, searching for "4th axis" or "Rotary table fixture" yields high-quality STEP files.
If you are looking for free files, these platforms are the industry standards for engineering and hobbyist models: 4th axis 3d model free download
These repositories are known for 3D printing, but a rising trend is "CNC Rotary" tags. GrabCAD is the largest community of professional engineers
: These search engines aggregate results across sites like Thingiverse and Printables. Searching for "4th axis rotary cnc" will yield thousands of printable attachments. Simple 4th Axis | 3D CAD Model Library | GrabCAD UCAM ROTARY URH 201 | 3D CAD Model Library | GrabCAD CNC Rotary tables gear - Download 3D CAD models for free : These search engines aggregate results across sites
Happy machining, and may your chips be blue and your bits be sharp.