While I agree with the other members who have said it's not worth more than twenty bucks, I encourage you to keep it if you think it's interesting and it makes you happy. I certainly have many bills in my own collection that have a special meaning to me and I wouldn't want to part with even if they're nothing special to others. Collecting is all about what you make of it.
That being said it is not particularly uncommon to find a 9999 bill. Trivial math will tell you that they are 1 in 10000, which makes it quite uncommon but certainly not unheard of. I go through that many bills in about six weeks and have found quite a few myself. Any more 9s though and I'd say you've got something special.
In fact I own a bill #9979999 that I found on my first day during the one time in my life I have visited Montreal, which I consider extremely special both from its number and the circumstances of finding it. Irrelevant to your question, but seeing your username, I had to share. Happy hunting.