Molly,
My first question is, "Is there a particular reason why the records can't start at ST4833 instead of ST121?" I know from personal experience, as a well-practiced OCD-er, that this would probably drive me nuts, also. However, a record number is a record number--that's all. The auto-number is used as an index to recognize unique records in the system, and nothing else.
Second, if there is a specific reason why the records must be sequential, then having the your record number be part of an auto-number sequence is ill-advised, because any deleted records will leave a gap in the auto-number sequence. Determine another method for calculating out the next sequential Record. This may require some thinking about how to do that, especially if it have letters and numbers (not to mention what would happen, based on your current convention when the number goes to ST1000 from ST999). All this can be done, and probably rather easily.
However, in order for us to effectively guide you to a solution, we would need some additional information.
BTW, based on your post, so far, I do not believe you are in "huge trouble"...
:-)