excel - Create a many-to-one sheet from a one-to-many sheet

Question

I have a sheet in Excel, Something like:

| Value1 | Data1 | Data1b | 1,3,4,8 |
| Value2 | Data2 | Data2b | 2       |
| Value3 | Data3 | Data3b | 6,7,8   |

I'd like to take that sheet and make another that divides up that final column into separate rows and keeps the other data in sync. So when the first sheet is updated, the second is also updated. And if a number is added to that final column in the first sheet, a new row is added to the second sheet.

The second sheet should look like this:

| Value1 | Data1 | Data1b | 1 |
| Value2 | Data2 | Data2b | 2 |
| Value1 | Data1 | Data1b | 3 |
| Value1 | Data1 | Data1b | 4 |
| Value3 | Data3 | Data3b | 6 |
| Value3 | Data3 | Data3b | 7 |
| Value1 | Data1 | Data1b | 8 |
| Value3 | Data3 | Data3b | 8 |

UPDATE : Below is the code I'm attempting to use. First of all, is what I have the best way? And is clearing then repopulating the right way to go about updating the second sheet? Finally, how do I make this automatically run when one updates the first sheet?

UPDATE : The only thing still not working is the sort at the end, does anyone have any idea why?

Private FROM_SHEET As String
Private TO_SHEET As String
Private START_ROW As Long
Private NUM_COL As Long

Sub oneToMany()

    FROM_SHEET = "Sheet1"
    TO_SHEET = "Sheet2"
    START_ROW = 2
    NUM_COL = 4

    Dim fromSheet As Worksheet
    Dim toSheet As Worksheet
    Dim newRow As Long

    Set fromSheet = Sheets(FROM_SHEET)
    Set toSheet = Sheets(TO_SHEET)

    toSheet.UsedRange.ClearContents

    newRow = START_ROW
    For i = START_ROW To fromSheet.Cells(fromSheet.Rows.Count, 1).End(xlUp).Row
        Dim col As String
        Dim nums() As String

        col = fromSheet.Cells(i, NUM_COL)
        nums = Split(col, ",")

        For Each num In nums
            fromSheet.Rows(i).Copy toSheet.Rows(newRow)
            toSheet.Cells(newRow, NUM_COL) = Trim(num) 'Should copy then overwrite?
            newRow = newRow + 1
        Next num
    Next

    'Sort not working
    toSheet.Range(toSheet.Cells(START_ROW, START_COL), toSheet.Cells(lastRow, lastCol)).Sort _
        key1:=toSheet.Range(toSheet.Cells(START_ROW, NUM_COL), toSheet.Cells(lastRow, NUM_COL)), _
        order1:=xlAscending, Header:=xlNo

End Sub

Answer 1

In answer to your first question if this is the only bit of data on the sheet then your clearcontents method is fine.

To automatically run look at the Worksheet_Change event on Sheet1. You can target the changes based on the cell changed....

Private Sub Worksheet_Change(ByVal Target As Range)

If Target.Row < 4 And Target.Column < 5 Then
    Call oneToMany
End If

End Sub