Linus Torvalds has tagged the fourth RC of the Linux 2.6.39 kernel. Early on in the 2.6.39 development cycle, Linus found the work this time around to be unusually calm and that continued even into last week's RC3 release. This week, however, there was more churn than usual and when compared to 2.6.39-rc3.
There's been some block layer plugging issues in the Linux kernel due to some changes that appears to have caused an infinite stream of on-disk notifications on CD-ROMs. Linus and the other developers have been sorting the issue out and everything should be settled, but it caused more activity than normal.
The Linux 2.6.39-rc4 kernel also has various other changes. There's some file-system changes, including for Btrfs, along with the usual variety of driver updates. There are some DRM/KMS updates to fix various outstanding regressions.
More on the Linux 2.6.39-rc4 kernel can be found in the release announcement at LKML.org.