Excel VBA run time error

When the following statement is executed the error below occurs…
        ColLast = .Cells(4, .Columns.Count).End(xlToLeft).Column
Run-time error ‘424’;
Object required

Can anyone helpwith this?
Thanks in Advance

Code...

Sub UpdateData()
    Dim ColLast As Long, RowLast As Long
    Dim AppendStr As String, Str As String
    Dim RowIndex As Long, ColIndex As Long
   

    With ActiveSheet.Name 'Sheet1
        ColLast = .Cells(4, .Columns.Count).End(xlToLeft).Column
        RowLast = .Cells(.Rows.Count, "D").End(xlUp).Row
        Dim StrArray() As String, i As Integer
               
        For RowIndex = 5 To RowLast
            AppendStr = ""
           
            i = 0
            For ColIndex = 2 To ColLast
                If .Cells(4, ColIndex) = "Regulatory" And .Cells(3, ColIndex).Interior.ColorIndex <> 43 Then
                    Str = .Cells(RowIndex, ColIndex)
                    If i = 0 Then
                        AppendStr = Str
                        ReDim Preserve StrArray(i)
                        StrArray(UBound(StrArray)) = Str
                        i = i + 1
                    ElseIf UBound(Filter(StrArray, Str)) <= -1 Then
                        AppendStr = AppendStr & ", " & Str
                        ReDim Preserve StrArray(i)
                        StrArray(UBound(StrArray)) = Str
                        i = i + 1
                    End If
                End If
            Next ColIndex
           
            .Cells(RowIndex, 1) = AppendStr
        Next RowIndex
       
    End With

End Sub