skiboot 6.6.4 was released on Thursday Oct 22nd, 2020. It replaces skiboot-6.6.3 as the current stable release in the 6.6.x series.
It is recommended that 6.6.4 be used instead of 6.6.3 version due to the bug fixes it contains.
Bug fixes included in this release are:
asm/head: fix power save wakeup register corruption
FSP/NVRAM: Do not assert in vNVRAM statistics call