Reiser4 was released for Linux 4.9.0 last weekend but then a revised patch series came out three days ago to fix some problems with this port to 4.9. With the new Reiser4 patches built against Linux 4.9.1, all should be well if you want to use this experimental file-system on the newest Linux kernel.
If you want to try out Reiser4 for Linux 4.9 or older kernels, visit the SourceForge site for the patches.
Reiser4 is still progressing on the feature front bit by bit. Edward Shishkin wrote that their RAID1/mirror support is looking stable and that it will be merged to stable soon. Next up they are working on implementing a distribution of read I/O requests against all available mirrors for speeding up read operations when using multiple devices. Those wishing to track the Reiser4 development code can use this GitHub repository.