You're looking for a PowerMill 4-axis post processor download. Here's some informative content to help you:
A 4-axis post processor is required when working with 4-axis CNC machines, which have an additional rotary axis (e.g., A, B, or C axis) compared to 3-axis machines. The 4-axis post processor ensures that the toolpath data is correctly translated into G-code, taking into account the rotary axis movements. powermill 4 axis post processor download
Autodesk PowerMill offers various post processors for different CNC machines and controllers. You can find a list of available post processors in the PowerMill documentation or on the Autodesk website. You're looking for a PowerMill 4-axis post processor