Jamieson, Fausset and Brown on Isaiah 50:4

4. Messiah, as "the servant of Jehovah" (Is 42:1), declares that the office has been assigned to Him of encouraging the "weary" exiles of Israel by "words in season" suited to their case; and that, whatever suffering it is to cost Himself, He does not shrink from it (Is 50:5, 6), for that He knows His cause will triumph at last (Is 50:7, 8).

learned--not in mere human learning, but in divinely taught modes of instruction and eloquence (Is 49:2; Ex 4:11; Mt 7:28, 29; 13:54).

speak a word in season--(Pr 15:23; 25:11). Literally, "to succor by words," namely, in their season of need, the "weary" dispersed ones of Israel (De 28:65-67). Also, the spiritual "weary" (Is 42:3; Mt 11:28).

wakeneth morning by morning, &c.--Compare "daily rising up early" (Jr 7:25; Mr 1:35). The image is drawn from a master wakening his pupils early for instruction.

wakeneth ... ear--prepares me for receiving His divine instructions.

as the learned--as one taught by Him. He "learned obedience," experimentally, "by the things which He suffered"; thus gaining that practical learning which adapted Him for "speaking a word in season" to suffering men (He 5:8).