Modules & VBA :: Function To Keep Numeric Value 4 Digits

I have the below function that works correctly. However, I would like the "Last_Nbr_Assigned" value to start at 0000 and increment by 1. So the very first number assigned would be 0001, then 0002, then 0003, and so on.

This number is automatically assigned so I haven't had luck with the validation properties. And if manually enter 0001 for the "Last_Nbr_Assigned", it will automatically cut off the 3 zeros, leaving me with 2 instead of the desired 0002.

Code:
Function NewRF() As String
Dim startRange_Var As Long, lastNum_Var As Long
On Error GoTo Err_Execute
'Retrieve last number assigned for RF
groupname_Var = Nz(DLookup("Group_Name", "Codes"), 0)

[Code] ....