Jamieson, Fausset and Brown on Isaiah 18:4

4. take ... rest ... consider--I will calmly look on and not interpose, while all seems to promise success to the enemy; when figuratively, "the sun's heat" and "the night dews" ripen their "harvest"; but "before" it reaches its maturity I will destroy it (Is 18:5; Ec 8:11, 12).

like a clear heat--rather, "at the time of the clear (serene) heat" [Maurer].

upon herbs--answering to "harvest" in the parallel clause. Maurer translates, "in the sunlight" (Job 31:26; 37:21; Ha 3:4).

like ... dew--rather, "at the time of the dew cloud." God's "silence" is mistaken by the ungodly for consent; His delay in taking vengeance for forgetfulness (Psa 50:21); so it shall be before the vengeance which in the last day shall usher in the restoration of the Jews (Is 34:1-8; 57:11, end of the verse, 2Pe 3:3-10).