Quite in contrast to Moses’s reticence to return to Egypt, the narrative in 4:27 appears to assume (or at least does not qualify) Aaron’s ready obedience to God’s command. Instead of explaining his divine commission and risking shame if it fails, Moses has told Jethro that he will go to Egypt to see if any of his brothers remained alive (4:18). Here God undermines that excuse, sending Moses’s brother to meet him while Moses is still in the wilderness at the mountain of God. God is well able to communicate with us even very specific information when we need that.
When did Aaron meet Moses at the mountain (4:27)? Perhaps it was before Moses started on the journey (4:18, 24) and surely before he returned to Egypt (4:20). Exodus 4:24, however, suggests that they met after the journey has begun (Aaron does not appear in that account); perhaps Moses and his family had started their journey from the side of Mount Horeb further from Egypt, the mountain lying between Egypt and where Jethro’s tents were currently located. Aaron’s sending was already announced in 4:14, but the meeting is finally mentioned here in 4:27 to prepare for the ministry of Moses and Aaron together in 4:28—5:1. (The awkward chronology of this section might reflect the stitching together of separate sources or stories, but, with many narrative critics, I am suspicious of our ability to reconstruct these very securely.)
Moses recounted to Aaron what the Lord had shown him (4:28). Even when we hear from God, he usually reveals to us only part of the message; God sent Aaron, but Aaron still needed to hear from Moses the details of God’s revelation to Moses. Again we see God’s right to choose as he sees fit. Aaron seems more obedient than Moses here, so why is Moses the main agent? Yet God knows what he will make of Moses, as well as knowing Aaron’s future weakness under distress (32:1-5). We are wise not to despise or be jealous of others’ gifts or callings; we should each do our best with the particular mission God has entrusted to us. (Compare similarly John 21:19-22, climaxing the apparent friendly rivalry between a young Peter and beloved disciple.)