This object reports the name of a candidate input clock source configured specifically for T4, or external output, clock selection, as distinct from the T0 sources used for the device's own internal system clock; a device can distribute a different, independently-selected timing signal outward than the one it uses internally. It matters because it lets an admin verify what timing sources are available to feed the T4 distribution path specifically, which may be a different pool of candidates than the T0 input table. Seeing this table populated with entries separate from the T0 input source table confirms the device supports independently sourced T4 output timing, useful when a downstream network segment needs a different reference than the device's own internal clock.