Exo 4:6-8 - It is interesting to see that all the three signs that God gave to Moses had symbolic affinities to the redemptive plan of God. The rod and the serpent, the leprous hand, water and blood, all these speak of human sin and divine redemption. I don't think this was accidental. But, what is more striking is the direct literal experience of Moses.
In this second sign, Moses' hand turns leprous, then is healed again. Leprosy was considered unclean in those days. It carried stigma and reproach. It was physically contagious and a threat to society. It symbolized hopeless alienation. Here, Moses experiences becoming unclean (of leprosy) and then becoming clean (through healing) in a moment. He becomes suddenly a man of reproach and then is restored to honour. In an instant, he becomes an outcaste and then is healed to be included. What went through the mind of Moses in the instant he took his hand out of his bosom and saw that he was now a leper? What went through his mind when he put it back in his bosom and took it out to see he was healed?
I think it is more like an internal catharsis, purging. This shocking moment of sudden pain should have freed him and healed him of his 40 years of guilt-ridden, reproach-filled life. His sudden intense experience of reproach and restoration, in a moment, must have delivered him from any sense of godless self-reproach.
It is interesting to see the divine move from rod to hand in sequence to the question, "What is in your hand?" For, even if Moses didn't have the rod, he had hands for God to use. No excuse is excuse enough before God.
Also, God's sovereignty is manifest here. We don't maintain as some of our brethren do that God's sovereignty includes human errors, that the sin of man was not beyond the sovereignty of God. We certainly believe that God is not the author of sin. But, at the same time we also know that the God who condemns us by the Law is also the God who has provided for us a way to be justified and be declared as clean in His sight by His grace.
The man called by God is clean, holy, and separated for the service of God; therefore, the missionary can say with Paul, "I labored... yet, not I but the grace of God that was with me." (1Cor.15:10).
Exo 4:9 - This was not the miracle of the plague that God intended for Egypt. In the plague, Moses had to strike the water with his rod (Exo.7:17); but, here he is told to take the water of the river and pour it on dry land, turning that water into blood. This sign is different from the first two in that it is a sign which is not exemplified here. In the former case, the rod was turned into a serpent and Moses' hand did become leprous as an example and proof of what God was about to do. However, turning water into blood is not given as an example. Of course, in one way, because the sign pertained to the Egyptian river. But, then, God didn't use an example of any water (say the bag of water Moses may have had) to illustrate the miracle. It seems that by this point, after the first two miracles, God is expecting Moses to move forward in faith with regard to the third. Also, this is the sealing miracle: it would be impossible for the people to not believe that God had appeared to Moses when Moses would have challenged the host of Egyptian poly-deities by turning the water of Nile into blood. The God who could challenge Nile (considered to be the source of all life) could not be just any Egyptian God; He had to be the God of Israel. The general and natural belief is that the water of the river, when it fell on the ground, made the ground fertile. Hapi (god of floods) was important to the Egyptians; for, the annual floods brought fertile soil. But, Moses was to take that same water and pour it on to the ground, turning the water into blood. In other words, the water that was considered to be life-giving now spelled death. This is not just a sign anymore, it is a proof of an anti-thesis; a proof of battle; a proof of conflict. The God of Israel was not actually the God who was sympathetic to the Egyptian gods. But, to challenge the Nile also meant to channel the life-source power of Egypt; to assert the supremacy of God's Kingdom over the powers of Egypt.
The missionary, on a deliverance mission, is not for a holiday trip; he is out for a spiritual battle, he is a challenge to the forces of darkness that hold humans in captivity (Eph.6:12; Exo.8:9; Lk.11:20). The missionary doesn't tackle pluralism by making the Gospel palatable to the poly-deities; the missionary speaks the truth in love (Eph.4:15) and brings all thought into captivity to the obedience of Christ (2Cor.10:5).
We are told that Moses did show these signs to the children of Israel when he and Aaron reached there (Exo 4:30) and they believed, though not for long.