SQL Server 2012 - Multiple named Instances with Always On
It doesn't really matter which one you pick. If it were my servers though, I would use the same instance names on each server. So INST1 would appear on SERVER1 and SERVER2, and then you'd AG INST1 with INST1. 

But I don't like stacking instances on a server as it makes it hard to troubleshoot and do maintenance. If these are virtual machines, setup another virtual machine to host the other instance, or in your case setup two more virtual machines.