CFW is already available but can only be installed if you have a compatible model and have not updated to OFW 4.91 yet (the latter condition will change soon, the former won't).
HEN for 4.91 is still under development. It will be released eventually. There's already HFW 4.91 but by itself HFW is useless so don't install it. If you are still on 4.90 with HEN then just wait until proper support for 4.91 is released. Or if you have a CFW compatible model you should consider switching.
