This read-only object identifies the fourth-priority timing reference source for the Synchronous Equipment Timing Subsystem (SETS) in Nokia/Alcatel-Lucent equipment. In systems supporting multiple clock sources (such as GPS, cesium oscillators, and network timing signals), the SETS maintains a priority list and falls back to lower-priority sources if higher-priority ones fail. Network operators use this configuration to ensure continuous clock synchronization and minimize timing glitches during reference source switchovers.