Auto incrementing PO number

Hi all. I'm new here, so I'm sorry if there have been a ton of these questions asked already. I searched a bit, but am still having difficulty with my problem. I have been trying many different VB codes to help my situation, but as of yet, haven't gotten it sorted. I'm very new to VB as well, so I don't understand a lot of things with coding, things need to spelled out a bit for me.
Ok, so here's my problem - I have a Purchase order template called Purchase Order form in Excel 2003. My actual spreadsheet I've titled Sales Order. In the cell G4 I have my purchase order number, which I would like to have auto increment for NEW purchase orders only, not the ones I've already done and saved as a new file name.
The problem I've encountered with the VB codes I've used thus far is that it does everything I want it to do the first time, but then after that the new purchase orders do not auto increment, or it auto increments on all my purchase orders, both new and previously saved ones. Getting really frustrated with this because it seems that other people are able to make the VB code work, but I'm not. As I've said, I'm not a VB genius, so I need everything spelled out please. I hope someone here can help. I've not had success on another forum.