Some thoughts...
- For a 4 vCPU FT VM you need Enterprise Plus licensing. The standard brings 2 vCPU
- The old legacy FT starts with vSphere 4.0 and there was no copy of the data (vDisks) needed. With the current implementation you have to specify a 2nd. location and FT creates a copy of the complete VM when you enable FT. There is only one vSAN Datastore per Cluster so iam not sure about the FT behaviour. If it works... you have 4 copies of your data.... so keep this in mind
- HA needs to be activated for FT VMs
- During a planed maintanence peroid you will not have data redundancy with only 3 hosts because there is no host left to evacuate the data to
- Think about going AFA instead of Hybrid when having such "small" capacity requirements. AFA is part of vSAN standard license
Sorry for not giving a clear answer. I will it test later i creating a FT VM is possible ... we have a couple of vSANs up and running.
Regards,
Joerg