Only 4 will be used for that particular virtual machine. However if you have other virtual machines, each one can have 1, 2, or 4 virtual processors and would consume additional cores. You can overcommit virtual processors to physical cores to some extent.
Now if SBS 2008 is the ONLY vm you are planning on running, it will not be able to exploit all of the processors unless you purchase Enterprise Plus level licensing.
VMware's chief strength is in its ability to share the resources available on the physical host among all of the VMs that you create to run on the box. The idea is if you need 10, 15, 20 servers - all needing 1, 2, whatever processors - you can put all of those servers onto ESX Servers and not have to buy 10, 15, 20 etc. physical servers.
