April 2013 CU for SharePoint Server 2013 is planned to be released in a couple of days.
In case you are only interested in April CU for SharePoint Foundation 2013 or Office Web Apps Server 2013 you can already download it from here:
The Full Server Packages for April 2013 CU are available through the following links:
As announced March 2013 PU for SharePoint 2013 defined a new baseline for all future SharePoint fixes - so ensure to install March 2013 PU before installing SharePoint 2013 April CU.
you can install SP1 over all pre and post SP1 fixes.
Thanks for your response, You are right on this because I did a POC on that.