Is this Oracle or MS SQL Server? They have different licensing in regards to CPU/vCPU =
As I recall the MS SQL EULA you will need to license the number of vCPUs - so you will have to buy a 4 COU license -
I'd strongly recommend you read Microsoft's Virtualization Licensing Guide. The licensing changed with SQL Server 2012 from CPU to core licensing and you may now either license the host or individual virtual machines (with a minimum of 4 cores per host/VM).
before MS SQL 2012 it was licensed per CPU(phyical) even if you only had 4 vCPU you still had to license it per scocket
now it is per core so if you have 4 vCPU you only need 4 licenses one for each vCPU
Re: SQL Server 2012 Downgrade to 2008 Licensing this link provides more detail.
As you can see from the different responses, licensing is very difficult. I'd recommend you either check the MS documentation for your specific requirements (e.g. which version may fit best, Standard, Enterprise or Datacenter) and/or have Microsoft confirm proper licensing before you purchase the licenses.
This is a typical consultants it depends question. you best bet would be to ask a MS license expert. There are other issues at play here to, things like do you have an enterprise SA agreement in place, this changes the rules as you need to license for the latest versions, even if your plan to downgrade your licenses to an earlier version.
In that case you would have to have 4 x core licenses for your server, regardless of your end operating state.