What are the support windows for SQL Server 2012 SP3 and SP4 respectively?

The lifecycle policy for Microsoft products is generally that the support for the previous Service Pack expires 12 months after the new Service Pack was released to the public.

So if the SQL Server 2012 Service Pack 4 is due in June 2017, then the Service Pack 3 will lose its support approx. June 2018.

Microsoft Support Lifecycle Site

If you search for SQL Server 2012 on the Lifecycle Support page you will see the results for the previous service pack Service Pack 2 and the following information for the current Service Pack 3:

                | Life Cycle Start Date | Mainstream Support | Service Pack Support End Date
----------------+-----------------------+--------------------+-------------------------------
Service Pack 2: | 10/06/2014            | Not Applicable     | 10/01/2017 
Service Pack 3: | 01/12/2015            | 11/07/2017         |

As you can see the newer Service Pack 3 limits the support of the previous Service Pack 2 to approx. 1 year after the newer Service Pack was introduced.

This is normally what happens with each newer Service Pack. Microsoft gives you a grace period of a year to upgrade to the newer Service Pack.

So you can expect (no guarantees from Microsoft) that the support for the older Service Pack 3 will expire one year after the introduction of Service Pack 4 (which would be June 2018).