OctaneRender® V4 requires a Nvidia driver supporting at least CUDA 9.1. On Windows, a recent enough Nvidia driver (http://www.nvidia.com/download/index.aspx?) is necessary.
Failure to install these driver versions may result in stability problems. The OctaneRender Team cannot provide support to users of different driver versions.
The CUDA driver is the part of the NVIDIA driver stack to which Octane uses. On Windows it is part of the NVIDIA graphics driver.
If you install any recent Nvidia graphics driver Windows, it will install a CUDA driver that supports CUDA 9.1. The CUDA Toolkit is only for development and does not have to be installed. The Toolkit does include a graphic driver, however, it may not be the latest.