ESXi hosts are licensed by CPU sockets, i.e. physical CPUs. So what you need in order to run FT (assuming you are using vSphere 6) is a license for vCenter Server - either Standard, or Foundation (for up to 3 ESXi hosts) - and 4 CPU Standard licenses (1 for each CPU). Please keep in mind that FT with Standard CPU licenses is limited to 2 vCPUs per FT-VM (see Compare VMware vSphere Editions | United States)
André