This seems to imply that I can run Everything 1.4 and 1.5 side by side. But some time has passed since that thread was active, so maybe things have changed since then?
Nothing has changed since.
You can still run Everything 1.4 and Everything 1.5 alpha side by side.
Everything 1.4 and Everything 1.5 alpha use a separate Everything Service.
Also: after the release of version 1.5, will the name for the service remain as "Everything 1.5a" service name? Or will default installs return to a service name of simply "Everything". (I can sense the potential for confusing no matter which choice is made with this matter.)
Once Everything 1.5 is in beta, the 1.5a instance will be removed.
You will not be able to run Everything 1.4 and Everything 1.5b side by side by default.
Same applies to the release version.
The Everything 1.5 Service will be renamed to
Everything Service
once in beta.
It is possible to use a custom instance in Everything 1.4 to install a service with a different name.
eg:
Everything-1.4.exe -instance "1.4"
will use a service named
Everything Service (1.4)
Everything 1.5 will only have one service, which manages all 1.5 instances.