It depends what kind of virtualization you want to have.
You can have a hypervisor which sits on top of an OS (Type 2). This means that you would need to run an operating system which would have software which is running the 4 VMs. In essence this means having one extra OS on the system, using up those precious resources.
Enterprise solutions use a hypervisor which speaks directly to the hardware (Type 1) but runs off a very small OS of sorts. This means that you would log into some sort of console on your computer and individually control which LPAR comes up and with what kind of settings. IBM has it's HMC (Hardware management console) which allows another computer on the same network as your server to connect to it and make changes to the VMs.
For the software type of virtualization, you can use VirtualBox (personal favorite) or VMware (more restrictive). These are most well known ones, there are tons more.
If you want something that is bare-metal, that is that speaks directly to the hardware, you can try Citrix Xen. With this category of hypervisor, money usually comes into play. Xen is however free under GPL.
Another cool thing about virtualization is that you can assign resource pools (for CPU usage and memory). A pool allows the OS to take what it needs in terms of resources, then put back what it doesn't for another OS to pick up. This is going to be key for you with a home computer with only 4GB of ram and 2 cores.