Because you are just clearing contents you dont need to loop through each cell within the Merged area so heres a better example. You may still want to include the other actions such as screenupdating, turning off events etc.

Sub BetterExample()
Dim Rng As Range, c As Range
Set Rng = Union(Range("Rng3"), Range("Rng4"), Range("Rng5"), Range("Rng6"))
For Each c In Rng
    If c.MergeCells Then
        If c.Address = c.MergeArea(1).Address Then c.MergeArea.ClearContents
    Else
        'do what ever to non merged cell
    End If
Next c
End Sub