Mine is four digits and begins with a 5 🙂  I've been in business in this city since 1977 and I don't recall but I think I might have begun using software in the mid 80's. Somehow I think my first computer may have been an apple and I think I had to purchase an IBM.

I was a heavy smoker in those days, I would complete a tax and while it was computing I could smoke half a cigarette. hahaha 

The software I began with was purchased by Lacerte, and more recently by Intuit.