Of those last four sequential-order digits, 0001 through 0100 designate prototype and one-off models, sample instruments and other special instruments; 0101 through 9999 designate all regular production models.
Gretsch used several different serial number schemes through the years.
Each of them can be used to accurately pinpoint the age of a guitar, if you have the correct information.
Unfortunately, correct information can be very difficult to come by.
By , by the way, we mean since 2003, which is when Gretsch serial numbering took its current form.