Sub Replace430()

Dim MyRange As Range
Dim RowCount As Long
Dim ColCount As Integer
Dim MyArray() As Variant

Dim i, j, k, percent30 As Long
Dim Count4 As Long
Const Replaced = 0   'Set Replaced Value
Const found = 18     'Set Find Value
Const Mycol = 3      'Correctly set the Column Number of Column in Range where 4 is to be checked
Set MyRange = Range("B2:F23")

RowCount = MyRange.Rows.Count
ColCount = MyRange.Columns.Count
'Get number of 4's againts 18
For i = 1 To RowCount
    If MyRange.Columns(1).Cells(i) = found Then
        For j = Mycol To Mycol
            If MyRange.Columns(j).Cells(i) = 4 Then
               Count4 = Count4 + 1
            End If
        Next j
    End If

Next i

ReDim MyArray(Count4 - 1, 2)
k = 0
For i = 1 To RowCount
    If MyRange.Columns(1).Cells(i) = found Then
        For j = Mycol To Mycol
            If MyRange.Columns(j).Cells(i) = 4 Then
               MyArray(k, 1) = i
               MyArray(k, 2) = j
               k = k + 1
            End If
        Next j
    End If

Next i

percent30 = 0.3 * Count4

Dim shufflearray()
ReDim shufflearray(Count4 - 1)
For i = 0 To Count4 - 1
    shufflearray(i) = i
Next i

'Shuffle the shufflearray() below


    Dim N As Long
    Dim Temp As Variant


    Randomize
    For N = LBound(shufflearray) To UBound(shufflearray)
        j = CLng(((UBound(shufflearray) - N) * Rnd) + N)

        If N <> j Then
            Temp = shufflearray(N)
            shufflearray(N) = shufflearray(j)
            shufflearray(j) = Temp
        End If
    Next N

'Use randomised values from shufflearray as array subscript to replace only 30% of 4's
For i = 0 To percent30 - 1
    MyRange.Columns(MyArray(shufflearray(i), 2)).Cells(MyArray(shufflearray(i), 1)).Value = Replaced
Next i


End Sub