Hide my invoice number I'm not a mathematician, so please forgive any ignorance.
I have a small business - I'm generating invoices incrementally. I'm currently on about invoice number 4000.
I guess I don't want my customers knowing how much business I'm doing (i.e. if they get an invoice for 4500, they know I've been doing a lot of business lately. However, if they get an invoice for 4010, they know things are slow).
So, my question is: how can I map say, 4500, to a guaranteed-to-be unique, human-readable/human-rememberable sequence?
e.g. 
4500 -> a48b82w
4501 -> b802aq2
4502 -> qi289a1
etc.
Is there a quick mathematical function that can do this? I have no idea...

Why make such a simple problem so complex?
Most e-commerce and invoice software supports starting numbers and increments, so just pick a large number and increment by another odd number. So something like:
1034578 = first invoice number
32876 = increment amount
So your invoices would follow 1034578 + 32876X like so:


*

*1067454

*1100330

*1133206

*etc.


For any person to know your invoice total they would need to determine the initial invoice number (improbable), and determine your increment (fairly easy).
