I am planning 1 web front end server and 1 application server in a Test and Production farms. So, I will have a total of 4 servers. Do I need 4 instances of SharePoint 2013 license Enterprise edition or is 1 license enough?
Yes, you need a license for each SharePoint server in a farm. There are licensing options for the Test environments, e.g. MSDN, but the restriction there is only users with MSDN licensing may access the Test environment.
Don't forget there are also SharePoint Standard and Enterprise User CALs. Each user using Standard features needs a SharePoint Standard User CAL. Each user using Enterprise features needs a Standard and Enterprise User CAL.
@Sam, you may also want to look at this link that shows various topologies and costs for SharePoint Enterprise and SQL Server - msdn.microsoft.com/en-us/library/dn305848.aspx Feb 7, 2016 at 0:10