I am installing SSRS on my SP 2013 farm. 2 web front ends and 2 app servers. I am confused as to what I need to install on what and if I need to install it on all servers. Run in SharePoint mode. Its more of confusion as to whether I need say for the WFE side I need to install the add-in on both WFE servers or if it can be just one and the same for the app server
The SSRS add-in must be installed on all SharePoint servers in the farm. As for the SSRS instance, that can be installed on any server in the farm, although you'll get better performance from an end user perspective by installing it on one or both FEs, should your FEs be capable of handling the additional load.
As for the addin, make sure to install the latest supported version for your combination of SSRS and SharePoint.