Auto Running Number in Excel Worksheet

Hi everyone.
Pls review on the attached PO Template, which I use it to issue Purchase Order and keep track on monthly purchases.
This template allow me generate a fresh PO with feature of auto "Naming" which is also the refernce number of the PO.
However, its only allow me to generate at maximum of 1000 POs in same worksheet with limited reference number to 3 digits, i.e. from POFM 001 ~ 1000. If i further generate the PO in this workbook for more that 1000, there incremental sequence number will just not working..

What I wish for improve for this template are as follows:-

1. Is there a way for me to start my running number which is more that 3 digits?
2. As I am creating a new workbook monthly (which is to avoid a heavy file size over time), how I can continue the running number from previous month in another new workbook for coming month?
3. I wish to have same PO refernce number (which is the same as worksheet name) in the body of PO template say in default cell "H4" when a new PO is created, how can I do this with VBA?

Hope someone could help...

Regards, CJ Yap