Turns out it's two, not four VMs. From macOS Sierra's EULA[0]:
> (iii) to install, use and run up to two (2) additional copies or instances of the Apple Software
within virtual operating system environments on each Mac Computer you own or control that is
already running the Apple Software, for purposes of: (a) software development; (b) testing during
software development; (c) using macOS Server; or (d) personal, non-commercial use.
> (iii) to install, use and run up to two (2) additional copies or instances of the Apple Software within virtual operating system environments on each Mac Computer you own or control that is already running the Apple Software, for purposes of: (a) software development; (b) testing during software development; (c) using macOS Server; or (d) personal, non-commercial use.
[0]: http://images.apple.com/legal/sla/docs/macOS1012.pdf