Macro to find duplicate value and replace adjacent cell with different value

Maybe:

Sub kosherboywww()
Dim i As Long
For i = Range("A" & Rows.count).End(3)(1).Row To 2 Step -1
    If Range("A" & i).Value = Range("F4").Value Then
        Range("B" & i).Value = Range("H4").Value
    End If
Next i
End Sub