Or HaChaim on Exodus 4:14/אור החיים על שמות ד׳:י״ד

1 ויתר אף ה׳ במשה, G'd became angry at Moses, etc. I believe that as a result of G'd's anger Moses did indeed remain afflicted with his stammer even after he undertook the mission in the end. The Torah says that G'd became angry במשה, at Moses; I understand that to be a reference to his body, i.e. it had an effect on his deformity. Our sages in Zevachim 102 who suggested that Moses referred to his older brother Aaron when he made the comment G'd found objectionable, believed that Moses who had been meant to also fulfil the function of High Priest was deprived of this function as a result of G'd's anger. The Talmud's comment is perfectly compatible with my own, as the words ויחר אף hinted at two things.

2כי דבר ידבר, "for he will certainly speak," etc. The reason that G'd repeated the word דבר is that G'd indicated that though Aaron was able to speak fluently now, i.e. דבר, he would also speak in the future, ידבר, though he would also become afflicted with a handicap similar to that of Moses. On the other hand, in line with our explanation of the consequences of G'd's anger (which had not been spelled out in detail as yet), the word דבר may refer to G'd's instruction to Moses, whereas the words ידבר הוא have to be read together and tell us that Aaron would do what Moses had refused to do. The emphasis on הוא implies also that whereas he, Aaron would display fluency of speech, Moses would not be cured.

3וגם הנה הוא יוצא לקראתך, "and he is also on the way to meet you, etc." If this is a continuation of G'd's praise of Aaron that he would be willing to speak although he too would stammer, this is additional praise for Aaron; the word וגם, "and also," is in place then. G'd is saying then that Aaron not only does not mind his handicap but he is glad to be able to perform a task G'd assigns to him. [I believe the reason the author says that Aaron suffered from a speech handicap is that he reads the words דבר־־ידבר as graphically describing the stutter. Ed.]

4The word וגם may also be understood in conjunction with Shabbat 139 where we are told that Aaron was granted the position of High Priest in recognition of his being happy that his younger brother had been appointed as the redeemer. When we consider that G'd had decided not to cure Moses of his speech defect because he had declined the request by G'd to assume leadership of the people, the word וגם would refer to the additional result of that refusal by Moses.

5The Torah may also have wanted to underline the virtues of Aaron and his humility. Normally, one could have expected him to be slighted for being appointed to be merely his younger brother's mouthpiece, but the Torah testified that far from feeling slighted, Aaron actually rejoiced over Moses' promotion. One of the lessons to be learned from this is that humility should not be carried to the point where one endeavours to escape fulfilling the will of the king (G'd). G'd had to tell Moses that Aaron would rejoice as the latter was not displaying this joy openly. The Torah could also have wanted to tell us that Aaron did display this joy on his face, but that G'd wanted Moses to know in addition that Aaron was equally joyful about Moses' appointment in his heart.