Psalm 23:4
4 Even though I walk through the darkest valley,
I will fear no evil,
for you are with me;
your rod and your staff, they comfort me.
Verse 4 is the transition piece of this chapter. It finishes up the metaphor of the shepherd before transitioning to the metaphor of the host in the rest of the chapter.
The darkest valley can also be translated "the valley of the shadow of death." Either way the meaning is the same—a dark place. This Psalm contrasts two dueling presences—the presence of God and the darkest valley. The presence of God is not only to be found in the green pastures and the still waters, it is also to be found in the darkest valley.
The psalmist declares that he will fear no evil in the darkest valley because YAHWEH is with him. It is precisely this assurance of God's presence with him in the darkest valley that dissolves his fear. Note that he transitions to the second person ("you") from the third person ("he") in verses 1-3. It is in the dark valley times of his life that the theological statements of who God is become a personal prayer, a direct communication... a relationship.
Continuing with the shepherd metaphor he says, "your rod and your staff, they comfort me." The rod and the staff imply the shepherds ability to protect the sheep and to guide them appropriately. The word translated "comfort" usually means something more along the lines of "courage". It doesn't just mean "to sympathize with". Instead it carries more the idea of "encourage". So the psalmist here is declaring that the protection and guidance of the Shepherd give him courage to walk through even the darkest valleys of his life.