Jamieson, Fausset and Brown on Zechariah 4:6

6. Not by might ... but by my Spirit--As the lamps burned continually, supplied with oil from a source (the living olive trees) which man did not make, so Zerubbabel need not be disheartened because of his weakness; for as the work is one to be effected by the living Spirit (compare Hag 2:5) of God, man's weakness is no obstacle, for God's might will perfect strength out of weakness (Ho 1:7; 2Co 12:10; He 11:34). "Might and power" express human strength of every description, physical, mental, moral. Or, "might" is the strength of many (an "army," literally); "power," that of one man [Pembellus]. God can save, "whether with many, or with them that have no power" (2Ch 14:11; compare 1Sa 14:6). So in the conversion of sinners (1Co 3:6; 2Co 10:4). "Zerubbabel" is addressed as the chief civil authority in directing the work.