I have 1 application insights and I've linked 4 applications to that single application insight.
When using Live Metrics can I see all the 4 applications at once just by using 1 application insights?
You will see all four applications with all instances from those applications.
If you upgrade Application Insights SDK to latest (Java, Node.js, .NET) then UX also will show "Roles" filter and will allow to zoom into a particular application.