I wouldn't use an autonumber for something that needs to be in incremental order always. Autonumbers are really only good for getting a UNIQUE number and not much else because of the possiblity that they will slide into random mode or you lose one when you start a record and then cancel, etc.
I would look into the DMax + 1 method here. And, there was just a post yesterday (I'll see if I can find it) where Paul (pbaldy) had responded about this and suggested the single record in a table method of this to keep users from colliding.
It will be 1, 2, 3, 4 and you can FORMAT it any way you like, so you can format to see 01, 02, 03, 04 (but with the leading zeros it will be text and not numbers).