Yes as this is a ESXi host limit - any supported configuration maximums for ESXi are the same regardless of whether running vSAN on top of this (and I am not aware of any exceptions to this).
Correct. Because vSAN is integrated with vSphere, a lot of of the maximums fall under vSphere (ESXi+vCenter), unless explicitly indicated under the vSAN section of the configuration maximums. For Stretched-clusters, it can become interesting, but luckily we have PFTT and SFTT that can help with this. See Cormac's blog Supporting Fault Tolerance VMs on vSAN Stretched Cluster - CormacHogan.com
Hope that helps
Thanks TheBobkin and GreatWhiteTec for your replies. I just wanted to double check that 4 is the max for vsan.