It should at least match performance of XFS in most workloads (similar features including extents, delayed alloc, etc. Search this site for the details you need.), but has the benefit of way more experienced devs than XFS ever will, and is home grown to the Linux kernel. Suffice to say, once the dust settles and ext4 has been incorporated into the major distros it will be far more stable and reliable than XFS could ever dream, even though the latter has been around for quite a while.
This seems to be how FOSS works, where the most popular projects attract the best devs and tend to take the lead and run with it even if it isn't [originally] the best technology. That isn't to say that niche projects don't exist, but for a general purpose FS, XFS really has no advantage over ext4. The more advanced XFS features like freezing are being ported to other FSes anyways.
In summery, ext4 is a worthy contender in the FS space and should handle most user's needs better than any of the other general purpose FS, at least until Btrfs stabilizes. Btrfs might not even benchmark as well as ext4, but it will allow for novel features like efficient/safe backup, Time Machine work-a-like, easy multi-volume management, and all the hype of ZFS and co. This tandem of ext4 and Btrfs should keep us content for a few years yet.