With vSMP you can run a VM with 1, 2 or 4 virtual CPUs (vCPU). When ESX schedules a CPU cycle for a VM, it has to find the same number of free physical CPU cores as there are vCPUs the VM. So for a 2 vCPU VM, ESX would have to find two free CPU cores and for a 4 vCPU VM ESX would find 4 free physical CPU cores. So to answer your one questions, you would require a server with at least 4 physical CPU cores to run a 4 vCPU VM.

So for vSMP each vCPU will execute on a seperate physical CPU core and those CPU cores must be free at the same time for the VM to get a CPU cycle. Here's a good paper to read through on the topic - http://www.vmware.com/pdf/vsmp_best_practices.pdf