What are the support windows for SQL Server 2012 SP3 and SP4 respectively?
SP4 for 2012 was announced recently and is due out next month.

See Microsoft's Support Lifecycle Policy documentation for end-of-support dates.  According to that document, SQL Server 2012 Service Pack 3 "mainstream support" ends on July 11, 2017.  Extended support for SQL Server 2012, including service pack 4, ends July 12, 2022.