This was a really silly one for me. Adding this here as it's one of the more popular threads on svc 404 issues.
I had in my Project Settings' \ Web \ Project URL, pasted:
http://blah.webservice.local.blahblah.com/Blah.svc
And for some unknown reason (having done this a thousand times) didn't spot straight away that the name of the .svc file was at the end.
DOH!
I had just pasted the address from my WCF test client and hadn't checked it sufficiently. What this did in the background was create an IIS application at the .svc address and I was getting nothing out of IIS. I couldn't work out how I couldn't even hit the .svc file.
Simple fix, obviously, just remove the application in IIS and change the project URL.
After almost 20 years at this, you can still make schoolboy errors / rookie mistakes. Hope this helps someone.