A.T. Robertson Commentary Revelation 9:4

"And it was said unto them that they should not hurt the grass of the earth, neither any green thing, neither any tree, but only such men as have not the seal of God on their foreheads." — Revelation 9:4 (ASV)

It was said (ερρεθη). First aorist passive indicative of ειπον.

That they should not hurt (ινα μη αδικησουσιν). Sub-final (object clause subject of ερρεθη) with ινα μη and the future active of αδικεω as in 3:9; 8:3. Vegetation had been hurt sufficiently by the hail (8:7).

But only such men as (ε μη τους ανθρωπους οιτινες). "Except (elliptical use of ε μη, if not, unless) the men who (the very ones who)." For this use of οστις see 1:7; 2:24; 20:4.

The seal of God upon their foreheads (την σφραγιδα του θεου επ των μετωπων). Provided for in 7:3f.. "As Israel in Egypt escaped the plagues which punished their neighbours, so the new Israel is exempted from the attack of the locusts of the Abyss" (Swete).