The official download for Autodesk T-Splines v4.0 r11183 for Rhino is no longer directly available on the main Autodesk website , as the product reached its end-of-life in 2017 www.rhino3d.com Current Download Options
If you hold a valid license, retrieve your installer from Autodesk archives or your backup drives. If you are starting a new project today, however, invest the time in learning Rhino’s SubD tools—they are the legitimate, supported evolution of the T-Splines dream. t-splines - v.4.0.r11183 download