It rather depends upon which OSs you are using. With 4 machines running you would really be best off with a quad core processor (or better) but if they are not very busy you might get away with a dual-core. The RAM calculation is easy, just add up the amount of RAM you would want if each of the VMs were a real computer and that's how much you need for the VMs - and then you need on top of that whatever the host OS requires.
So if each of the VMs was running Windows 7 you'd need at least 10GB, allowing a minimum of 2GB for each installation plus another 2 for the host. (Don't forget that if you are running Microsoft guests you need a licence for each VM.) XP or Linux guests are much easier of memory - say 1GB minimum per VM.