Ex 4:19 And the LORD said to Moses in Midian, "Go, return to Egypt; for all the men who sought your life are dead."

 20 Then Moses took his wife and his sons and set them on a donkey, and he returned to the land of Egypt. And Moses took the rod of God in his hand.

21 And the LORD said to Moses, "When you go back to Egypt, see that you do all those wonders before Pharaoh which I have put in your hand. But I will harden his heart, so that he will not let the people go.

At this point, Moses, was still apparently fearful that God had to assure him that all the men who sought his life were already dead. There was a degree of unbelief in verse 18, and so, here, in verse 19, we read of his apparent fearfulness.  

Through God's encouragement, he managed to shoo away his fears and doubts that he boldly went to Egypt. The fact that he took his entire family in verse 20 demonstrates this. 

God gave him an advance notice as to what reception his ministry would have back in Egypt in verse 21. He should not be discouraged if the result would not turn out favorable.