Depends on what workload you will be running inside the VM.
For example, a Windows Server 2012 R2 Lab VM with just the AD and DNS roles enabled runs just fine with a single vCPU, configured with 1GB of RAM (and only averages just under 400MB active, so can probably be reduced to half a GB of RAM.)
A client VM like yours will probably need more than that, though, as most desktop environments tend to run more apps on them than a server (note, I say "tend to" as there are usually exceptions to everything.) I would struggle to see the need for 4GB though. My Windows 7 and 8.1 VMs run with 2GB and always have spare memory available. They all have Evernote, MS Office 2013 (including Visio), and Firefox/Chrome installed in them. All of which have at least one or two Office apps running (commonly Word/Excel), Evernote, and either Firefox or Chrome running. Never ran out of RAM on them yet.
Whatever you decide to use, it will reduce the amount of available RAM to OS X by that same amount. So, plan accordingly. Leave at least 2GB of RAM minimum for OS X (I personally leave at least 4GB.)
(My VMware lab environment consumes over 10GB on my MBP with a nested ESXi cluster and associated VMs. So, 16GB RAM is a must for me, but I'm not the "average" user that Apple targets with their stuff, either.)