I have a spreadsheet that has 44,383 rows. I am trying to use the following
macro to fill in the blanks for columns A:D. I am getting a REF error. I
know it is the number of rows because the macro works for spreadsheets that
do not have as many rows. Is there any modification to this code that will
make the macro fill down for 44,383 rows:

Sub FillBlanks2()
Dim Rng As Range
On Error Resume Next
Set Rng = ActiveSheet.UsedRange.Columns("A:D")
With Rng
.SpecialCells(xlCellTypeBlanks).FormulaR1C1 = "=R[-1]C"
.Copy
.PasteSpecial xlPasteValues
End With
Set Rng = Nothing

End Sub