The two operative verbs in Ps 121:4 are:
- נוּם (num) = to be drowsy or slumber, Ps 76:5, 121:3, 4, Isa 5:27, 56:10, Nah, 3:18.
- יָשֵׁן (yashen) = to sleep, eg, Gen 2:21, 41:5, Lev 13:11, 26:10, Deut 4:25, Judges 16:19, 1 Sam 26:12, 1 Kings 19:5, Job 3:13, Ps 3:5, 4:8, 13:3, 121:4, Prov 4:16, Eccl 5:12, SS 5:2, 7:9, Isa 5:27, Jer 51:39, 57, Eze 34:25, Hos 7:6.
Both are uncontroversial verbs whose meaning is undisputed. They are always translated literally.
This distinction in meaning in Ps 121:3 &4 appears that God, our protector, neither dozes/slumbers nor sleeps. That is, God's protection of Israel is unrelenting because God is always fully alert. The amplified Bible attempts to make this clearer by offering:
Behold, He who keeps Israel Will neither slumber [briefly] nor sleep [soundly].
Almost all English translation offer something almost the same as
... will neither slumber nor sleep.
Thus, the use of these two verbs in synthetic parallelism is a perfect example of Hebrew poetic repetition to add emphasis.