How to generate an alphanumeric tree changes with criteria?

I'm working on excel sheet template used for SAP System and I have 2 columns looks like below:

Code:

Column C               Column E

Level                  Element Code

3                      ABCD.01.01.01               

4                      ABCD.01.01.01.01

4                      ABCD.01.01.01.02

4                      ABCD.01.01.01.03

3                      ABCD.01.01.02

4                      ABCD.01.01.02.01 'I Want to Restart Numbering Here

4                      ABCD.01.01.02.02

4                      ABCD.01.01.02.03

I succeeded in level 3 to be automated in the whole sheet by Macro as below

Code:

Sub AutoNumber3()



Dim Rng, C As Range

Dim Lrow As Long

Dim i As Integer

Lrow = Cells(Rows.Count, 1).End(xlUp).Row

Set Rng = Worksheets("Union").Range("C2:C" & Lrow)

i = 1

For Each C In Rng.Cells

If C.Value = 3 Then

For i = 1 To i Step 1

C.Offset(0, 2).Value = "ABCD.01.01." & i

Next i

End If

Next C

End Sub

and I used the same for level 4 as below

Code:

Sub AutoNumber4()

Dim Rng, C As Range

Dim Lrow As Long

Dim i As Integer

Lrow = Cells(Rows.Count, 1).End(xlUp).Row

Set Rng = Worksheets("Union").Range("C2:C" & Lrow)

i = 1

For Each C In Rng.Cells

If C.Value = 4 Then

For i = 1 To i Step 1

C.Offset(0, 2).Value = "ABCD.01.01.01" & i

Next i

End If

Next C

End sub

I want to Restart the numbering of level 4 from 1 each time the cells values in the level column = 3 by using Do Until the next C.Value = 3, I = 1 But I can not put it correctly in the Autonumber4 procedure

Your help is highly appreciated since this sheet may reach to 50000 or 100000 rows which is impossible to fill them manually

Thanks, Regards

Moheb Labib