simple Account number creator in autohotkey

I require an autohotkey script that simply generates the next highest four digit number that hasn't already been used. They will be used as account numbers in MYOB accounting software. The number will be produced when windows + a is pressed i.e. #a in autohotkey. That number will also be added to a csv file of numbers which have already been used. The csv file must be independently editable using a program such as xcel so I can correct errors and start reusing dormant account number if I need to. The csv file is to be called [login to view URL] and I will give you the directory of where kept. So, if the csv file contained the numbers 0001 0002 0003 0005 Then the script would give me 0004 when I press windows + a and would also add 0004 to the list of numbers already used. Then the next time I pressed windows + a it would give me 0006 at the cursor and so on. Once the script is produced I want to be able to simply paste it into my [login to view URL] If you can think of a way of doing this without necessarily using autohotkey I am open to suggestions. Cheers Ben