If you have a Host with 4GB RAM and Quad core processor, those are your hosts resources. Your host will be running a OS and also an application that will virtualize other OS's which is your workstation.
Now, after the hosts OS consumption of resources, you will be left with around half the resources to run virtual machines on workstation.
So you will be able to run one VM with 2GB RAM and 2 cores. If you create another VM, it will just starve without resources and sometime you will not be able to even power them on. All the resources (Memory, CPU and disk) that virtual machines run on are all taken from your host. If you have abundant resources on host the VM's run fine too.