You will not be able to do this using ColStyles. Instead, try handling the QueryCellInfo event on the embedded grid and handling things there.
AddHandler GridListControl1.Grid.QueryCellInfo, AddressOf grid_QueryCellInfo
Private Sub grid_QueryCellInfo(ByVal sender As Object, ByVal e As GridQueryCellInfoEventArgs)
If e.ColIndex = 4 AndAlso e.RowIndex > 0 Then
e.Style.Format = "dd/MM/yyyy hh:mm tt"
End If
End Sub