Talk Truths
There are two original words translated to "plummet": "stone" and "alloy" and it refers to Jesus being placed on the spiritual house of the saints, considering the context in v.7.Zechariah 4:7XWaiting for response This verse tells us that God and all the people will rejoice when Jesus is placed in the spiritual house.
Verses 6-7
Zechariah 4:6-7XWaiting for responseZerubbabel was the leader of the returning exiles to Israel from Babylon and built the temple Ezra 3:8.Ezra 3:8XWaiting for response God said that the temple would not be built by man's power or might, but by his power v.6.Zechariah 4:6XWaiting for response The great mount (task / difficulty / obstacle) will be made flat through God's spirit and as the last stone (headstone) is placed it will be blessed with calls for grace to it v.7.Zechariah 4:7XWaiting for response What a task it was to return from exile to a land with hostiles and rebuild their capital city alongside the temple of God!
These prophecies are also fulfilled in Jesus and the spiritual house of the saints e.g. 1 Peter 2:5.1 Peter 2:5XWaiting for response This is an even greater task which spans over millennia.