Windows 10 is an iteration of Windows 8.1 … and that sequence goes all the way back to Windows 1.0…It doesn’t mean that Windows 10 is windows 8.1
And obviously UE4 was finished and in use before its release. I would be willing to bet that Unity was also finished and in use before its 1.0 release. The difference here is that Unity 5 IS Unity 4 IS Unity 3 and so on… built up over the last 11 years. This means that the code base and foundation of Unity will be significantly older than UE4’s
UE4 has beautiful source code, either indicating its youth, or some ongoing continuous effort to refactor UE4 to keep source clean.