How do I include 64 bit python with ArcGIS 10.4 install?

If you only have access to ArcGIS Desktop or Engine, you need to install the Background Geoprocessing for 64bit, check Python scripting with 64-bit processing at the ArcGIS Blog. If you have ArcGIS Server, the 64bit Python will be automatically installed.

A Help page on that: What is 64-bit Background Geoprocessing?

The download for the 64 bit geoprocessing installation is not freely available. I found it in my.esri.com under My Organizations -> Downloads -> ArcGIS for Desktop 10.4 -> ArcGIS for Desktop Background Geoprocessing (64-bit).