ext4 (especially with the new disk format and new features) was only declared 'stable' by it's developers in this last kernel release. In spite of the fact that it shares three characters of it's name with more common filesystems there is a huge amount of new code in it that has not had extensive real-world testing yet. I think that we're only in the 6-9 month range since the developers started trusting their own data to it.
the fact that Fedora (a bleeding edge distro) is considering using it for it's next release (in 4 months or so) just means that they think that it's good enough to test.
this is a FAR cry from saying that it's a reliable filesystem that is production ready.
NTFS speed may be terrible, but it's been around long enough to have let experience find the bugs that the developers missed.