Does Django consists of a migration concept without south app?
If not why didn't they include it in Django 1.4? (Since, it's a must have).
No, it doesn't. South is considered as the 'de facto' standard for database migration in Django. Further reading can be found in the Django development wiki.
From the version 1.2 release notes:
Contrib-01 (Add South to contrib) - general consensus - including that of South's lead developer - is that South isn't yet ready to be a bonafide part of Django.
This, most probably, is still valid. South has its limitations, it still has gaps that could be considered as flaws when being a django.contrib module.
That said, I want to add: south still is extremely useful.